Fig. 97.—Leitz’s Dissecting Microscope.
Fig. 97.—Leitz’s Dissecting Microscope.
The coarse adjustment is made by rack and pinion, and the fine adjustment by micrometer screw, the head of which is provided with a scale reading1â„100mm. The draw-tube is also cut and ruled to millimetre scale. The sub-stage has rack and pinion movement, and is arranged for the Abbe condenser and iris diaphragm. Thisis attached to the upper stage by means of a set pin, which fixes and retains it in position after perfect centring. By removing the pin, the sub-stage can be either detached or swung aside by pressing a button. In short, this microscope is in all respects well furnished and fitted with the requisite complex mechanism necessitated by modern high-class technicological work.
Leitz’s students’ microscope, with sliding body, micrometer screw fine adjustment, concave mirror, two eye-pieces and two objectives, ¾ inch and1â„8inch, in mahogany case, costs £3 10s.Leitz’s dissecting microscope, with a heavy foot and rests, is fitted with two aplanatic lenses, magnifying × 10, × 20 diameters.
Reichert and Seibertadhere to the same model as that of Zeiss, and therefore require only a brief notice. Their microscopes are characterised by substantial workmanship, suitable construction, and exact centring. The coarse adjustment is obtained in the usual way by rack and pinion, the fine by micrometer screws, which work easily, and are protected against wear and tear by having their working surfaces hardened. The stands of the better class instruments have micrometer screws graduated, and draw-tubes cut to millimetre scale. Their mechanical stages and sub-stages and accessories are in every way well finished; stage forceps, tests, and an assortment of cover glasses and slides being added. Their first-class microscopes are sent out in mahogany boxes.
On going through the continental makers’ catalogues, it will be noticed that their well-equipped microscopes are rather more costly than that of their Englishconfreres. It is understood Messrs. Baker and Watson are the constituted agents for these opticians.
Nachet’s Microscope, a new form of which was first seen at the Antwerp Exhibition 1892, is very solidly built, and has all the qualities necessary for histological work. The stage rotates about the optic axis, and carries a movable slide holder. The coarse adjustment is by rack and pinion movement, the fine by the new system of micrometer screw (described in the journal of the Royal Microscopical Society of 1886), with divided head indicating the1â„400part of a mm. The plane and convex mirror is mounted on a jointed arm. The draw-tube is divided into millimetres. The illuminating system, consisting of a wide-angled Abbe condenser(N.A. 1·40) with iris diaphragm, is raised or lowered by rack and pinion screws. The iris diaphragm, being mounted on a wheel, is worked by a tangent screw, which by a very slight movement causes the aperture of the diaphragm to pass from the centre to the periphery of the condenser. Altogether the arrangement of the sub-stage is novel, and the instrument is extremely well arranged and adapted to modern requirements.
Nachet and Hartnack, of Paris, hold an almost equal rank as makers of first-class microscopes, and in point of excellence of workmanship fairy rival those of our English makers.
Fig. 98.—Nachet’s Class Demonstrating Microscope.
Fig. 98.—Nachet’s Class Demonstrating Microscope.
There are very many other London and Continental makers of microscopes besides those especially mentioned, who have well-sustained reputations as opticians, and who, from want of space, I have been obliged to pass over. Messrs. Newton’s Students’ Microscope must be mentioned with respect. It is a good and useful instrument, has a firm stand with a reversible (rotatory) body movement, which seems to ensure steadiness when brought into the horizontal position for micro-photographic purposes. There are other opticians whose microscopes have stood the test of time—Messrs. Collins, Crouch, &c. It may, however, be taken as a well-established fact that those opticians known to manufacture the more highly-finished models also produce the more serviceable forms of students’ class-room, and other microscopes.
The microscope required for bacteriological studies should be perfect in all its parts. With regard to the choice of an instrument, it is very much a matter of price, since the most perfect is usually the most costly; I shall therefore proceed to give a typical example of the instrument in use in a bacteriological laboratory. The microscope should possess the following qualifications, all of which are absolutely necessary for the study of such minute objects as bacteria and other micro-organisms.
“The typical bacteriological microscope should be well equipped with objectives of sufficiently high magnifying power, and with a special form of illuminating apparatus; while the mechanical arrangements for focussing should act with the greatest smoothness and precision; the stage, also, should be wide enough to admit of the examination of plate cultivations.â€
We will consider these several points and recommendationsseriatim, commencing with the stand.
Messrs. Watson & Sons’ Van Heurck model stand so well answers the several conditions laid down by an experienced teacher of bacteriology, that I have no hesitation in presenting it to my readers as a typical instrument, one in every way worthy of the high praise it has already received from those who have worked with it, and whose judgment may be relied upon in every way. The microscope is fully described among Messrs. Watson’s instruments, page 108.
The Stand.—A good firm stand is undoubtedly of the first importance for all high-class work. The steadiness of the instrument and its entire freedom from vibration depends largely upon the form of the stand. I am glad to find Dr. Crookshank in accord with me as to the Ross-Jackson model, one which, in my opinion, has not been entirely superseded by models of a more recent date. Indeed, the latest improvement effected in the Ross-Jackson form, in which attention has been given to the spreading-out of the feet, has converted it into as solid and firm a stand as Powell’s; it is equally free from vibration when placed in the horizontal position.
There are, however, four different forms of stands—the tripod; the plate with double columns; the single column ending in a plate or a bent claw; and the horse shoe. The tripod stand, with cork feet,is by far the steadiest form of model. The single upright pillar support should unquestionably be condemned, as it admits of considerable vibration, and is most inconvenient for laboratory work. The heavy horse-shoe form is compact and firm, and the weight of it can hardly be considered an objection.
The Tubular Bodyis from eight to ten inches in length, to which is added a draw-tube with an engraved millimetre scale. By extending the draw-tube greater magnification is obtained, but since this is at the cost of definition it should hardly ever be employed in the examination of bacteria.A Triple Nose-pieceis doubtless a convenience, saving time which is otherwise spent in replacing objectives of different magnifying powers; there is also less risk of injuring them.Focusshould be obtained by means of a rack and pinion coarse adjustment, together with the most approved kind of fine adjustment. The sliding tube cannot be recommended, as the motion may be stiff, encouraging the use of force, which in turn may result in the objective being brought violently into contact with the specimen, thus doing injury to the lens or damage to the preparation; or it may get too loose and readily slip out of focus.
The Stageshould be flat and rigid, either rectangular or circular, so long as it is sufficiently large to accommodate plate cultivation. A removable mechanical stage is of great advantage for working with high powers, as a motile bacterium can be constantly kept in view, while one hand is engaged in working the fine adjustment; it may also be employed as a finder, if engraved with a longitudinal and vertical scale, and provided with a stop. The mechanical stage must be removable, so that the stage proper may be free from any attachments when required for the examination of cultures.
Diaphragms.—The plan of using a series of separate discs of different sizes should be avoided, as they are easily lost, and bacteriological investigations may have to be made under conditions in which it is difficult to replace them. A better plan is a revolving plate with apertures of different sizes, but the most convenient form is theiris diaphragm.
The Sub-stage Condenseris as necessary in biological work as in the objective—in fact, the condenser and the objective should be considered as forming one piece of optical apparatus; the microscope must be regarded as incomplete without it.
It is by thesub-stage condenserthat the rays of light are concentrated at one point, or on one particular bacterium; for the best definition it is essential that there should be mechanical arrangements for accurately centring and focussing the condenser. All this will be explained and enlarged upon under “Practical Optics.â€
In the historical review presented to my readers on the evolution of the modern microscope, I have for the most part relied upon my long and close association, extending over a period of upwards of half a century, with microscopy. I need hardly say I could have very much extended my remarks with pleasure and profit had space permitted, and thereby much increased the number of names of manufacturers, who have well-established reputations for the quality of their work, and whose instruments, more or less complete in design, realise the wants of students and of that large class of present-day workers engaged in microscopical pursuits to whom economy of outlay is almost a first consideration. No valid reason, however, can be assigned for splitting up, as some writers do, the several forms of microscopes into some six different classes, which implies inferiority in mechanical details or finish, whereas the difference wholly consists in luxurious appliances to save time, and in accessories for special work or original research. Before bringing these remarks to a close, it is my wish to direct the student’s attention to one or two points of importance in connection with the use of the instrument, viz.: variations in body-lengths of microscopes, especially between those of English and of Continental manufacture. Theoptical-standardmeasurement adopted in this country for the body-tube-length is 10 inches; and for itsmechanical, 8¾ inches. That of Continental opticians is, optical-tube-length 7·08 inches, or 180 mm.; the mechanical, 6·3 inches = to 168 mm.
Professor Abbe constructed an apochromatic immersion objective especially for the English optical tube-length of 10·6 inches (= to 270 m.m.), and mechanical tube-length somewhat less in measurement. This may be taken to mean a slight increase in the standard value of the tube, and therefore the addition of the rack-and-pinion to the draw-tube, now generally made a part of the microscope, is certainly of some practical value. This difference, however, when working with the English body-tube of 10 inches, may be discarded; it is, in fact, only where the shorter Continental body is in use, that sosmall a difference of tube-length exercises a disturbing effect over adjustment. Moreover, an object placed on the stage of the shorter body microscope will not be seen with the same distinctness by the draughtsman should he wish to make use of thecamera lucida.
Theopticaltube-length of the body is measured from the back lens of the objective to the front lens or principal focus of the eye-piece; themechanicaltube-length from the end of nose-piece of objective to the top lens of the eye-piece.
The Hartnach Students’ Model Microscope.
The Hartnach Students’ Model Microscope.
It is almost unnecessary to say that the eye-piece forms a most important part of applied optics in the microscope. It is an optical combination designed to bring the pencil of rays from the objective to assist in the formation of a real or virtual image before it arrives at the eye of the observer. Greater attention has been given of late years to the improvement of the eye-piece, since flatness of field much depends upon it. Opticians have therefore sought to make it both achromatic and compensatory.
There are several forms of eye-pieces in use, some of which partake of a special character, and these will receive attention in their proper places. It is, however, customary among English opticians to denote the value of their several eye-pieces by Roman capitals, A, B, C, D, and E. Continental opticians, on the other hand, have a preference for numerals, 1, 2, 3, 4, 5 and 6, or more, and by which they are recognised.
The eye-piece in more general use is that known as theHuyghenian(Fig. 99); this came into use upwards of two centuries ago. It was constructed by Christian Huyghens, a Dutch philosopher and eminent man of science, secretary to William III.
It was made for the eye-piece of a telescope he constructed with his own hands, and it has been in constant use as the eye-piece of the microscope for nearly two centuries. It consists of two plano-convex lenses, with their plane surfaces turned towards the eye, and divided at a distance equal to half the sum of their focal lengths—in other words, at half the sum of the focal length of the eye-glass and of the distance from the field-glass at which an image from the object glass would be formed, a stop, or diaphragm, being placed between the two lenses for the reason about to be explained.Huyghens himself appears to have been quite unaware of the value of an eye-piece so cleverly constructed.
Fig. 99.—Huyghenian Eye-piece A, the dotted lines show position of lenses.
Fig. 99.—Huyghenian Eye-piece A, the dotted lines show position of lenses.
It was reserved for Boscovich to point out that, by this important arrangement, he had corrected a portion of the chromatic aberration incidental to the earlier form of eye-pieces. LetFig. 100represent the Huyghenian eye-piece of a microscope,f fbeing the field-glass, ande ethe eye-glass, andl m nthe two extreme rays of each of the three pencils emanating from the centre and ends of the object, of which, but for the field-glass, a series of coloured images would be formed fromr rtob b; those nearr rbeing red, those nearb bblue, and the intermediate ones green, yellow, and so on, corresponding with the colours of the prismatic Spectrum.
The effect described, that of projecting the blue image beyond the red, over-correcting the object-glass as to colour, is purposely produced; it is also seen that the imagesb bandr rare curved in the wrong direction to be seen distinctly by the convex eye-lens; this then is a further defect of the compound microscope made up of two lenses. But the field-glass, at the same time that it bends the rays and converges them to foci atb′ b′andr′ r′, also reverses the curvature of the images as here shown, giving them the form best adapted for distinct vision by the eye-glasse e. The field-glass has at the same time brought the blue and red images closer together, so that they produce an almost colourless image to the eye. The chromatic aberration of lenses has been clearly explained in a previous chapter. But let it be supposed that the object-glass had not been over-corrected, that it had been perfectly achromatic; the rays would then have appeared coloured as soon as they had passed the field-glass; the blue rays of the central pencil, for example, would converge atb′′, and the red rays atr′′, which is just the reverse of what is required of the eye-lens; for as its blue focus is also shorter than its red, it would require that the blue image should be atr′′, and the red atb′′. This effect is due to over-correction of the object-glass, which removes the blue focib bas much beyondthe red focir ras the sum of the distances between the red and the blue foci of the field-lens and eye-lens; so that the separationb ris exactly taken up in passing through those two lenses, and the several colours coincide, so far as focal distance is concerned, as the rays pass the eye-lens. So that while they coincide as to distance, they differ in another respect—the blue image is rendered smaller than the red by the greater refractive power of the field-glass upon the former. In tracing the pencill, for instance, it will be noticed that, after passing the field-glass, two sets of lines are drawn, one whole and one dotted, the former representing the red, and the latter the blue rays. This accidental effect in the Huyghenian eye-piece was pointed out by Boscovich. The separation into colours of the field-glass is like the over-correction of the object-glass—and opens the way to itscomplete correction. If the differently-coloured rays were kept together till they reached the eye-glass, they would still be coloured, and present coloured images to the eye. The separating effected by the field-glass causes the blue rays to fall so much nearer the centre of the eye-glass, where, owing to its spherical figure, the refractive power is less than at the margin, so that spherical error of the eye-lens may be said to constitute a nearly equal balance to the chromatic dispersion of the field-lens, and the blue and red raysl′andl′′emerge nearly parallel, presenting a fairly good definition of a single point to the eye. The same may be said of the intermediate colours of the other pencils. The eye-glass thus constructed not only brings together the imagesb′ b′,r′ r′, but it likewise has the most important effect of rendering them flatter, and assisting in the correction of chromatic and spherical aberration.
Fig. 100.—Huyghenian Eye-piece.
Fig. 100.—Huyghenian Eye-piece.
Fig. 101.—Ramsden’s Eye-piece.
Fig. 101.—Ramsden’s Eye-piece.
The later form of the Huyghenian eye-piece is that of the late Sir George Airy, the field-glass of which is a meniscus with the convex side turned towards the objective, and the eye-lens a crossed convex with its flatter side towards the eye. Another negative eye-piece is that known as theKellner, or orthoscopic eye-piece. It consists of a bi-convex field-glass and an achromatic doublet eye-lens. This magnifies ten times, but it in no way compares with the Huyghenian in value. Neither does it afford the same flatness of field.
TheRamsden, or positive eye-piece, is chiefly employed as a micrometer eye-piece for the measurement of the values of magnified images. The construction of this eye-piece is shown inFig. 101, a divided scale being cut on a strip of glass in1â„100ths of an inch, every fifth of which is cut longer than the rest to facilitate the reading of the markings, and at the same time that of the image of the object, both being distinctly seen together, as in the accompanying reduced micro-photograph of blood corpuscles,Fig. 102.
The value of such measurements in reference to the real object, when once obtained; is constant for the same objective. It becomes apparent, then, that the value of the divisions seen in the eye-piece micrometer must be found with all the objectives used, and carefully tabulated.
It was Mr. Lister who first proposed to place on the stage of the microscope a divided scale of a certain value. Viewing the scale as a microscopic object, he observed how many of the divisions on the scale attached to the eye-piece corresponded with one or more of a magnified image. If, for instance, ten of those in the eye-piece correspond with one of those in the image, and if the divisions are known to be equal, then the image is ten times larger than the object, and the dimensions of the object ten times less than that indicated by the micrometer. If the divisions on the micrometer and on the magnified scale are not equal, it becomes a mere rule-of-three sum; but in general this trouble is taken by the maker of the instrument, who furnishes a table showing the value of each division of the micrometer for every object-glass with which it will be employed.
Fig. 102.—Blood Corpuscles and Micrometer, magnified 1·3500.
Fig. 102.—Blood Corpuscles and Micrometer, magnified 1·3500.
Mr. Jackson’s simple and cheap micrometer is represented inFig. 103. It consists of a slip of glass placed in the focus of the eye-glass, with the divisions sufficiently fine to have the value of the ten-thousandth part of an inch with the quarter-inch object-glass, and the twenty-thousandth with the eighth; at the same time the half, or even the quarter of a division may be estimated, thus affording the means of attaining considerable accuracy, and may be used to supersede the more complicated and expensive screw-micrometer, being handier to use, and not liable to derangement in inexperienced hands.
The positive eye-piece affords the best view of the micrometer, the negative of the object. The former is quite free from distortion, even to the edges of the field; but the object is slightly coloured.The latter is free from colour, and is slightly distorted at the edges. In the centre of the field, however, to the extent of half its diameter, there is no perceptible distortion, and the clearness of the definition gives a precision to the measurement which is very satisfactory.
Fig. 103.—Jackson’s Eye-piece Micrometer.
Fig. 103.—Jackson’s Eye-piece Micrometer.
Short bold lines are ruled on a piece of glass,a,Fig. 103, to facilitate counting, the fifth is drawn longer, and the tenth still longer, as in the common rule. Very fine levigated plumbago is rubbed into the lines to render them visible; they are then covered with a piece of thin glass, cemented by Canada balsam, to prevent the plumbago from being wiped out. The slip of glass thus prepared is secured in a thin brass frame, so that it may slide freely into its place.
Slips are cut in the negative eye-piece on each side, so that the brass frame may be pressed across the field in the focus of the eye-glass, as atm; the cell of which should have a longer screw than usual, to admit of adjustment for different eyes. The brass frame is retained in its place by a spring within the tube of the eye-piece; and in using it the object is brought to the centre of the field by the stage movements; the coincidence between one side of it and one of the long lines is made with great accuracy by means of the small screw acting upon the slip of glass. The divisions are then read off as easily as the inches and tenths on a common rule. The operation, indeed, is nothing more than the laying of a rule across the body to be measured; and it matters not whether the object be transparent or opaque, mounted or unmounted, if its edges can be distinctly seen, its diameter can be taken.
Previously, however, to using the micrometer, the value of its divisions should be ascertained with each object-glass; the method of doing this is as follows:—
Place a slip of ruled glass on the stage; and having turned theeye-piece so that the lines on the two glasses are parallel, read off the number of divisions in the eye-piece which cover one on the stage. Repeat this process with different portions of the stage-micrometer, and if there be a difference, take the mean. Suppose the hundredth of an inch on the stage requires eighteen divisions in the eye-piece to cover it; it is plain that an inch would require eighteen hundred, and an object which occupied nine of these divisions would measure the two-hundredth of an inch. Take the instance supposed, and let the microscope be furnished with a draw-tube, marked on the side with inches and tenths. By drawing this out a short distance, the image of the stage micrometer will be expanded until one division is covered by twenty in the eye-piece. These will then have the value of two-thousandths of an inch, and the object which before measured nine will then measure ten; which, divided by 2,000, gives the decimal fraction ·005.
Enter in a table the length to which the tube is drawn out, and the number of divisions on the eye-piece micrometer equivalent to an inch on the stage; and any measurements afterwards taken with the same micrometer and object-glass may, by a short process of mental arithmetic, be reduced to the decimal parts of an inch, if not actually observed in them.
In ascertaining the value of the micrometer with a deep objective, if the hundredth of an inch on the stage occupies too much of the field, then the two-hundredth or five-hundredth should be used and the number of the divisions corresponding to that quantity be multiplied by two hundred or five hundred, as the case may be.
The micrometer should not be fitted into too deep an eye-piece, as it is essential to preserve good definition. A middle-power Kellner or Huyghenian is frequently employed; at all events, use the eye-piece of lower power rather than impair the image.
The eye-lens above the micrometer should not be of shorter focus than three-quarters of an inch, even with high-power objectives.
The Ramsden Eye-piece.—The cobweb micrometer is the most efficient piece of apparatus yet brought into use for measuring the magnified image. It is made by stretching across the field of the eye-piece two extremely fine parallel wires or cobwebs, one or both of which can be separated by the action of a micrometer screw, the trap head of which is divided into a hundred or more equal parts, whichsuccessively pass by an index as the milled head is turned, shown inFig. 104. A portion of the field of view is cut off at right angles to the filaments by a scale formed of a thin plate of brass having notches at its edges, the distances between which correspond to the threads of the screw, every fifth notch (as in the previous case) being made deeper than the rest, to make the work of enumeration easier. The number of entire divisions on the scale shows then how many complete turns of the screw have been made in the separation of the wires, while the number of index points on the milled head shows the value to the fraction of a turn, that may have been made in addition. A screw with one hundred threads to the inch is that usually employed; this gives to each division in the scale in the eye-piece the value of1â„100th of an inch. The edge of the milled head is also divided into the same number of parts.
Micrometer scale to drop into Eye-piece.Fig. 104.—Ramsden Screw Micrometer Eye-piece.
Micrometer scale to drop into Eye-piece.
Micrometer scale to drop into Eye-piece.
Micrometer scale to drop into Eye-piece.
Fig. 104.—Ramsden Screw Micrometer Eye-piece.
In Watson’s Ramsden screw micrometer,Fig. 104, the micrometer scale (seen detached) is ruled on a circular piece of glass, and this, by unscrewing the top, is dropped into its place, and one of the wires, both being fixed, is set a little to the side of the field, the teeth of the screw being cut to1â„100ths, and the drum giving the fractional space between the teeth to1â„100ths, so that the1â„10000th of an inch can be read off. This micrometer eye-piece is constructed entirely of aluminium, a decided advantage, being so much lighter than brass to handle.
In the screw micrometer of other makers, other modifications are found. An iris diaphragm being placed below the web to suitthe power of the eye-piece employed, a guiding line at right angles to the web is sometimes added. Care should be taken to see that when the movable web coincides exactly with the fixed web, the indicator on the graduated head stands at zero.
The Compensating Eye-piece.—The very important improvements effected in the construction of the objective naturally led up to an equally useful change for the better in the eye-piece.
All objectives of wide aperture, from the curvature of their hemispherical front lenses, show a certain amount of colour defect in the extra-axial portion of the field, even if perfectly achromatic in the centre. Whether an image be directly projected by the objective, or whether it be examined with an aplanatic eye-piece, colour fringes may be detected, possibly in an increasing degree towards the periphery. This residual chromatic aberration has at length been very nearly eliminated by the aid of the compensating eye-piece.
The construction of compensating eye-pieces is somewhat remarkable, since they have an equivalent error in an opposite direction—that is, the image formed by the red rays is greater than that corresponding to the blue rays; consequently, eye-pieces so constructed serve to compensate for the unequal magnification produced by different coloured rays, and images appear free from colour up to the margin of the field.
Zeiss’s compensating eye-pieces are so arranged that the lower focal points of each series lie in the same plane when inserted in the body-tube of the microscope; no alteration of focus is therefore required on changing one eye-piece for another. This of itself is not only an advantage but also a saving of time, while the distance between the upper focal point of the objective and the lower one of the eye-piece, which is the determining element of magnification, remains constant.
Fig. 105.—A sectional view of Zeiss’s Compensating series of Eye-pieces, ½ the full size.A.—Plane of the upper edge of the tube.B.—Lower focal plane of eye-pieces, with their lensesin situ.
Fig. 105.—A sectional view of Zeiss’s Compensating series of Eye-pieces, ½ the full size.
A.—Plane of the upper edge of the tube.B.—Lower focal plane of eye-pieces, with their lensesin situ.
A.—Plane of the upper edge of the tube.
B.—Lower focal plane of eye-pieces, with their lensesin situ.
The ordinary working eye-pieces, Huyghenian and others, commencing with a magnification of four diameters, are so constructed that they can be conveniently used, as we are accustomed to use them in England, with high powers, Zeiss’s Nos. 12 and 18 compensating eye-pieces being adapted for use with his lower power apochromatic lenses of 16 and 8 mm. The numbering of the eye-pieces is carried out on the plan originally proposed by Professor Abbe—that is, thenumber denotes how many times an eye-piece, when employed with a given tube-length, increases the initial magnifying power of the objective, and at the same time furnishes figures for their rational enumeration. It is on this basis that the German compensating eye-pieces have been arranged in series, and in agreement with their magnifying power and distinctive numberings of 2, 4, 6, 8, 12, 18. Of these several eye-pieces, 12 is found to be the most useful. The magnification obtained by combining a compensating eye-piece with any apochromatic objective is found by multiplying its number by the initial magnification of the objective, as given in the following proof:—An objective of 3·0 mm. focus, for example, gives in itself amagnification of 83·3 (calculated, for the conventional distance of vision, 250 mm.); eye-piece 12 therefore gives with this objective a magnification of 12 × 83·3 = 1000 diameters. The classification, however, of these eye-pieces, as furnished by Abbe, is dependent upon increase in the total magnifying power of the microscope obtained by means of the eye-piece as compared with that given by the objective alone. The numbering, then, denotes how many times an eye-piece increases the magnifying power of the objective when used with a given body-tube; the proper measure of the eye-piece magnification; and, at the same time, the figures for rational enumeration.
Fig. 106.—B and C Achromatic Eye-pieces.
Fig. 106.—B and C Achromatic Eye-pieces.
Compensating eye-pieces have been introduced for the correction of certain errors in high-power objectives—those made with hemispherical fronts. All such lenses, whether apochromatic or not, are greatly improved by the compensating eye-piece, but the dry objective and the lower powers are certainly deteriorated. The lower power compensating eye-pieces are Huyghenian, the higher are combinations, with no field-lens, and therefore in working act as a single or positive eye-piece. This is of importance to those who work with low powers—the older forms of objectives.
Messrs. Watson and Swift have adopted a new formula for their series ofachromatic eye-pieces, whereby their magnification and flatness of field are improved. These also bear a constant ratio to the initial power of their objectives.
The compensating eye-pieces of these makers are constructed on the same principle as those of Zeiss’s for the correction of errors of colour in the marginal portion of the field, and consequently are in every way as effective as those of Continental manufacture. Figs. 106, 107,and 108 show in dotted outline the form and position of the several lenses combined in these eye-pieces.
Projection Eye-piecesare chiefly used in micro-photography, and for screen demonstrations. The cap of this eye-piece is provided with a spiral adjustment for focussing, the diaphragm being placed in front of the eye-lens, an essential arrangement for obtaining an accurate focus. The ring seen below the cap,Fig. 108, is graduated so that the rotation for distance of screen may be carefully recorded.
Fig. 107.—The Compensating Eye-piece.Fig. 108.—Projection Eye-piece.
Fig. 107.—The Compensating Eye-piece.
Fig. 107.—The Compensating Eye-piece.
Fig. 108.—Projection Eye-piece.
Fig. 108.—Projection Eye-piece.
Schmidt’s goniometer positive eye-piece, for measuring the angles of crystals, is so arranged as to be easily rotated within a large and accurately graduated circle. In the focus of the eye-piece a single cobweb is drawn across, and to the upper part is attached a vernier. The crystals being placed in the field of the microscope, care being taken that they lieperfectly flat, the vernier is brought to zero, and then the whole apparatus turned until the line is parallel with one face of the crystal; the frame-work bearing the cobweb, with the vernier, is now rotated until the cobweb becomes parallel with the next face of the crystal, and the number of degrees which it has traversed may then be accurately read off.
Goniometer.—If a higher degree of precision is required, then, the double-refracting goniometer invented by the late Dr. Leeson must be substituted. With this goniometer (Fig. 109) the angles of crystals, whether microscopic or otherwise, can be measured. Ithas removed the earlier difficulties incident to similar instruments formerly in use. Among other advantages, it is capable of measuring opaque and even imperfect crystals, beside microscopic crystals and those in the interior of other transparent media. It is equally applicable to the largest crystals, and will measure angles without removing the crystal from a specimen, provided only the whole is placed on a suitable adjusting stage. The value of the goniometer depends on the application of a doubly refracting prism, either of Iceland spar or of quartz, cut of such a thickness as will partially separate the two images of the angle it is proposed to measure.
Dr. Leeson strongly insisted on the importance of the microscope in the examination of the planes of crystals subjected to measurement, as obliquity in many cases arises from not only conchoidal fractures, but also from imperfect laminæ elevating one portion of a plane, and yet allowing a very tolerable reflection when measured by the double refracting goniometer.
Fig. 109.—Leeson’s Goniometer.
Fig. 109.—Leeson’s Goniometer.
Microscopes for crystallographic and petrological research are now specially constructed for measuring the angles of crystals.
Erector eye-pieces and erecting prisms are employed for the purpose of causing the image presented to the eye to correspond with that of the object. They are also helpful in making minute dissections of structure; the loss of light, however, by sending it through two additional surfaces is a drawback, and impairs the sharpness of the image. Nachet designed an extremely ingenious arrangement whereby the inverted image became erect; he adapted a simple rectangular prism to the eye-piece. The obliquity which a prism gives to the visual rays when the microscope is used in the erect position, as for dissecting, is an advantage, as it brings the image to the eye at an angle very nearly corresponding to that of the inclined position in which the microscope is ordinarily used.
Fig. 110.—Pan-aplanatic Achromatic Objectives.
Fig. 110.—Pan-aplanatic Achromatic Objectives.
The Achromatic Objective, of all the optical and mechanical adjuncts to the microscope, is in every way the most necessary, as well as the most important. The ideal of perfection aimed at by the optician is a combination of lenses that shall produce a perfect image—that is, one absolutely perfect in definition and almost free from colour. The method resorted to for the elimination of spherical and chromatic aberration in the lens has been fully explained in a former chapter. It will now be my endeavour to show the progressive stages of achromatism and evolution of the microscope throughout the present century.
It is almost as difficult to assign the date of the earliest application of achromatism to the microscope as to that of the inception and many modifications of the instrument in past ages; indeed, the question of priority in every step taken in its improvement has been the subject of controversy.
Among the earlier workers in the first decade of this century will be found the name of Bernardo Marzoni, who was curator of the Physical Laboratory of the Lyceum of Brescia. He, an amateur optician, it has come to light, in 1808 constructed an achromatic objective, and exhibited it at Milan in 1811, when he obtained the award of a silver medal for its merits, under the authority of the “Institute Reale delli Scienzo.†Through the good offices of the late Mr. John Mayall one of Marzoni’s objectives, which had been carefully preserved, was presented to the Royal MicroscopicalSociety of London in 1890.20This objective is a cemented combination, with the plane side of the flint-lens presented to the object. This was an improvement of a practical kind, and of which Chevalier subsequently availed himself. In 1823 Selligue, a French optician, is credited with having first suggested the plan of combining two, three, or four plano-convex achromatic doublets of similar foci, one above the other, to increase the power and the aperture of the microscope. Fresnel, who reported upon this invention, preferred on the whole Adam’s arrangement, because it gave a larger field. Selligue subsequently improved his objective by placing a small diaphragm between the mirror and the object.
In this country, Tully was induced by Dr. Goring to work at the achromatic objective, and his first efforts were attended with a success quite equal to that of Chevalier’s. Lister on examining these lenses said:—“The French optician knows nothing of the value of aperture, but he has shown us that fine performance is not confined to triple objectives.†Amici, the amateur optician of Modena, visited this country in 1827 and brought his achromatic microscope and objectives, which were seen to give increase of aperture by combining doublets with triplets. The most lasting improvement in the achromatic objective was that of Joseph Jackson Lister, F.R.S., the father of Lord Lister, and one of the founders of the Royal Microscopical Society of London.
Lister’s discoveries at this period (1829) in the history of the optics of the microscope were of greater importance than they have been represented to be. That he was an enthusiast is manifest, for, being unable to find an optician to carry out his formula for grinding lenses, he at once set to work to grind his own, and in a short time was able to make a lens which was said to be the best of the day.
Lister, in a paper contributed to the proceedings of the Royal Society the same year, pointed out how the aberrations of one doublet could be neutralised by a second. He further demonstrated that the flint lens should be a plano-concave joined by a permanent cement to the convex crown-glass. The first condition, he states, “obviates the risk of error in centring the two curves, and the second diminishes by one half the loss of light from reflection, which is very great at the numerous surfaces of every combination.â€These two conditions then—that the flint lens shall be plano-concave, and that it shall be joined by some cement (Canada balsam) to the convex—may be taken as the basis for the microscopic objective, provided they can be reconciled with the correction of spherical and chromatic aberration of a large pencil.
Andrew Ross was not slow to perceive the value of Lister’s suggestions and in 1831 he had constructed an object-glass on the lines laid down by Lister,Fig. 112;a a′representing the anterior pair,mthe middle, andpthe posterior, the three sets combined forming the achromatic objective, consisting of three pairs of lenses, a double-convex crown-glass, and a plano-concave of flint.
Fig. 111.—Lister’s double-convex crown and plano-concave flint cemented combination.Fig. 112.—Andrew Ross’s ¼-inch Objective.
Fig. 111.—Lister’s double-convex crown and plano-concave flint cemented combination.
Fig. 111.—Lister’s double-convex crown and plano-concave flint cemented combination.
Fig. 112.—Andrew Ross’s ¼-inch Objective.
Fig. 112.—Andrew Ross’s ¼-inch Objective.
Lister proposed other combinations, and himself made an object-glass consisting of a meniscus pair with a triple middle, and a back plano-convex doublet. This had a working distance of ·11 and proved to be so great a success that other opticians—Hugh Powell, 1834; James Smith, 1839—made objectives after the same formula.
The publication of Lister’s data proved of value in another direction: it stimulated opticians to apply themselves to the further improvement of the achromatic objective. Andrew Ross was one of the more earnest workers in giving effect to Lister’s principles and a short time afterwards found that a triple combination, with the lenses separated by short intervals, gave better results. In the accompanying diagram the changes made in the combination of the objective from 1831, and extending over a period of about twenty years from this date, are shown.
Each objective, from the ½-inch to the1â„12-inch, is seen to be built up of at least six or eight different fronts, the back combinations being a triplet formed of two double-convex lenses of crown glass with an intermediary double concave lens of flint-glass.
Fig. 113.—Combinations of Early Dry Objectives.A, Double-convex lens;B, Plano-concave;C, Bi-convex and plano-concave united; shown in their various combinations, as atD, form the 3-in., 2-in. or 1½-in.; atE, 1-in. and2â„3-in.; and atF, the ½-in.,4â„10-in., ¼-in. and1â„25-in. objectives.CombinationDwas for many years known as the Norfolk Objective.
Fig. 113.—Combinations of Early Dry Objectives.
A, Double-convex lens;B, Plano-concave;C, Bi-convex and plano-concave united; shown in their various combinations, as atD, form the 3-in., 2-in. or 1½-in.; atE, 1-in. and2â„3-in.; and atF, the ½-in.,4â„10-in., ¼-in. and1â„25-in. objectives.
CombinationDwas for many years known as the Norfolk Objective.