Fig. 26.--Hand-fed Arc LampFig. 26.—Hand-fed Arc Lamp.
The whole question of optical adjustments has, however, been left over for a future chapter, as it more or less applies to whatever illuminant is used.
The illustration shows a lamp arranged for continuous current, the upper carbon, which must be connected to thepositivewire, being larger than the lower (the negative), and very slightly behind it. The light from a continuous current arc lamp comes chiefly from this upper or positive carbon,which 'craters' as it is used, and this arrangement has the effect of radiating the light in the direction required (Fig. 27).
The positive carbon is usually of the 'cored' type, that is provided with a core of softer carbon, as this assists the 'cratering' action, while the negative is generally used 'solid,' that is homogeneous right through.
The arc has to be 'struck' in the first place by touching the carbons together for a moment by the mechanical means provided, and then separating them to the working distance, which is approximately ⅛ inch. They must then be maintained at that distance by 'feeding' as they slowly burn away, and this 'feeding' in arc lamps for lantern work is usually done by hand, as in the lamp illustrated in Fig. 26, but may be done by an automatic arrangement, as will be described later.
Fig. 27.Fig. 27.
The current is really carried across the arc byconvection, or in other words conducted by a bridge of white hot carbon particles, which continually stream across from the positive carbon to the negative, and this bridge, while conducting the current, interposes a very considerableresistance(otherwise it would not of course become hot).
A certain potential or tension is therefore necessary if a given current is to be maintained, and this potential has to be greater the longer the arc and also (though not in direct proportion) the smaller the carbons.
When, however, everything is in the best proportion,i.e.length of arc, size of carbons, and current passing, the potential at the arc lamp terminals required is approximately 45 volts, and this may be taken as a fixed figure for any current.
The length of arc to give the best results may also be takenas approximately fixed at ⅛ inch, and thevariablefactor for different currents as required is provided by altering the sizes of carbons employed.
The error must not be made, however, of assuming that an E.M.F. of 45 volts is sufficient to work an arc lamp, as the minimum in practice is at least 65 volts, and 100 or even 200 volts are advantageous.
I have come across more than one private generating installation where the innocent owner has put in a dynamo for 45 or 50 volts, depending upon some carelessly written statement that this is sufficient.
Whya higher E.M.F. is required can be simply explained.
Take for instance an average hand-fed arc lamp as used for lantern work and consuming, say, 10 ampères.
Take also, as a fact, the statement given above that the necessary E.M.F. at the actual terminals of the arc lamp may be accepted as a constant at 45 volts, and reverting to the equation given on page 40, C = E / R, and substituting these figures we get—
Current (10 ampères) = E (45 volts) / R (Resistance of Arc).
Current (10 ampères) = E (45 volts) / R (Resistance of Arc).
Current (10 ampères) = E (45 volts) / R (Resistance of Arc).
It is therefore obvious that under these exact conditions the resistance or back E.M.F. of the arc, as it is termed, must equal 4.5 ohms.
Now suppose the lamp left for a few seconds unattended, while the carbons are burning away and the arc is lengthening; in a very few moments the resistance will have increased, owing to the greater distance between the carbons, and we will suppose it to have become 5 ohms instead of 4.5.
The current passing will now be 45 / 5 = 9 ampères only.
In other words, a very slight lengthening of the arc has reduced the current, and therefore the light, by 10 per cent.
Not only so, but 45 volts being needed to maintain an arc ofnormal length, it is insufficient to maintain a longer one, and in practice the effect of leaving an arc under these conditions to itself for even a few seconds is that itgoes out, to the annoyance of the lecturer and the confusion of the operator.
It is justpossibleto work an arc lamp with a total E.M.F. of 45 volts by giving one's whole attention to it and never taking the hand off the feeding handle; but in practice no one with any experience would attempt it. The arc would almost certainly go out several times during the exhibition.
Now, take an example of a similar arc lamp consuming 10 ampères but worked from a supply of 200 volts.
Our equation C = E / R must then obviously become
C (10 ampères) = E (200 volts) / Total Resistance (20 ohms).
C (10 ampères) = E (200 volts) / Total Resistance (20 ohms).
C (10 ampères) = E (200 volts) / Total Resistance (20 ohms).
The resistance of the arc itself being the same as before, viz. 4.5 ohms, it is obviously necessary to put anextrafixed resistance equal to 15.5 ohms in series with it in order to make up the total of 20 ohms.
Nowleave the arc unattended until the resistance of 4.5 ohms has again become 5 ohms; the only effect is that our current, instead of remaining at 10 ampères, has become 200 / 20.5 or 9.8 nearly, a difference which is imperceptible.
This is not all, for it is an elementary rule in electrical science that the total E.M.F. of any circuit distributes itself along that circuit in proportion to the distribution of resistance.
In other words, our original E.M.F. of 200 volts will so distribute itself as to reserve, so to speak, an E.M.F. of 45 volts for the arc, while the resistance of this remains at 4.5 ohms, but directly this resistance increases, the E.M.F. at the arc lamp terminals automatically rises, and therefore the actual diminution in current is even less than the figures above quoted.
Should the arc tend to 'break' or go out, the resistance across it automatically becomes infinite and thewhole200 volts is at that moment available to prevent the occurrence.
Under these conditions, therefore, the operator can safely leave the arc for many minutes at a time. In carrying out experimental work I have often left the lantern, walked up to the screen, discussed results with a friend, and walked back, and the arc has shown no signs of misbehaviour whatever.
Fig. 28.--ResistanceFig. 28.—Resistance.
In practice any current from 100 volts to 250 volts may be considered as satisfactory for lantern work with a suitable resistance. Less than this involves feeding the arc rather frequently, and more may give a nasty shock, should the operator inadvertently touch a live wire, though I have worked an arc lamp on a current of as much as 500 volts.
Theresistanceusually consists of a suitable length of wire of high resistance (Iron, German Silver, or those alloys known as Platinoid, Eureka, Manganin, Beacon, &c., are most commonly used) wound in spirals on a frame, and is generally supplied adjustable (Fig. 28), so that more or less current may be used as desired. These resistances get pretty hot in use, and care must be taken that they are placed where they cannot scorch woodwork, &c., and in cases where the lantern is a fixture it is a good plan to have the resistance bolted up against a wall once and for all. The resistance may be placed anywhere in the circuit, so long as the current passes through it, then through the arc lamp (orvice versâ), and back to the otherpole of the supply main; it does not matter in the least whereabouts it comes.
In cases, however, where one pole of the supply main isearthed, it is a good thing to place the resistance in the 'live' side, as this keeps the arc lamp within 45 volts of earth potential while it is working, to the comfort of the operator should he touch a terminal or wire, though with an ordinary lighting main there is no real fear of a dangerous shock in any case.
Theamount of current requireddepends of course on the size of the sheet, length of the hall, and density or otherwise of the slides; but it is usually accepted in practice that the efficient light from a continuous current arc lamp equals 100 candles per ampère, and therefore a 10-ampère arc will give 1000 candles. This is sufficient for all ordinary halls and slides, but where these latter are very dense, as for example with the Lumière three-colour process, as much as 20 or 25 ampères may be required.
In these cases some special precautions must be taken for keeping the slides cool, or the result may be disastrous, but this is a question that will be referred to in a later chapter. A current of 10 ampères is pretty safe for all ordinary slides, and may be taken as the normal current used in large halls, though in arranging for the wiring it is as well to stipulate for at least 12 or even 15 ampères, especially as there must necessarily be a momentary increase of current at the instant the arc is 'struck.'
Varieties of Hand-fed Arc Lamps.—The pattern of hand-fed arc lamp illustrated in Fig. 26 is only typical of many of the same general design, and there are others in which the design itself is fundamentally different. Of these the 'Scissors' arc lamp made by several firms deserves mention on account of its simplicity and cheapness. As its name implies, the mechanism resembles a pair of scissors, the carbons being attached to the ends of a pair of levers hinged together(Fig. 29). In this lamp centring movements are usually dispensed with, the arc being clamped on to a tray pin as in the case of a limelight jet. This is not, of course, so convenient, and a further disadvantage of this pattern arc lamp is that the feeding process gradually alters the position and angle of the carbons. In fact, the one great merit of the lamp is cheapness, and where expense is an object, it should certainly be considered.
Fig. 29.--Scissors Arc LampFig. 29.—'Scissors' Arc Lamp.
Yet another arc lamp deserving of mention is the 'Parallel,' a name again very aptly chosen, as the two carbons are either exactly parallel to each other or very slightly inclined. In the former case the arc has to be 'struck' by touching the ends of the carbon rods with a piece of metal or carbon. Of the actual manipulation of this lamp I have had very little practical experience, but I have heard it well spoken of, though I believe it has so far only been made for currents of 5 ampères or so.
Yet another type which must not be ignored is the 'Right-angled' pattern (Fig. 30), a name again self-descriptive. The horizontal carbon is the positive, and the vertical thenegative, and this lamp again is made by several manufacturers in slightly different forms.
This pattern lamp is in my experience the best of all forsmallcurrents, say, of 5 ampères or so, but inferior to Fig. 26 for currents of 10 ampères or more. This last remark perhaps hardly applies toalternatingcurrents, which, however, I have not yet discussed. I cannot conclude this brief category of arc lamps without referring to theenclosedpattern, of which the 'Westminster' is perhaps the best-known and most popular (Fig. 31).
Fig. 30.--Right-angled Arc LampFig. 30.—'Right-angled' Arc Lamp.
This is a lamp of the right-angled type, but the arc burns in a cylindrical glass chamber, not air-tight, but partially so. After burning a few minutes the oxygen in this chamber becomes used up and its place is taken by carbonic-acid gas and other products of combustion, after which the carbons burn away very much more slowly, and therefore require feeding at much greater intervals.
This lamp again is chiefly made for small currents not exceeding 5 ampères (and can therefore be used from any ordinary lamp socket), and for a moderate-sized hall is on thewhole as cheap, efficient and simple a lamp as any I am acquainted with. It can be supplied with or without mechanical centring movements as required, and is usually sent out with its own resistance for the particular current on which it is to be used, so that it only requires connecting up to the nearest lamp socket, and is ready for use.
Fig. 31.--Westminster Arc LampFig. 31.—'Westminster' Arc Lamp.
It isnotsufficient for anything larger than a 12-foot sheet or for working at a greater distance than, say, 40 feet, but within these limits the lamp, and in factanygood 5-ampère arc lamp, will be found quite satisfactory and saves the expense of putting in a special cable.
Automatic Arc Lamps.—Arc lamps for lantern work in which the feeding is done automatically are also made. Like hand-fed lamps, they vary in exact design, but all, or practically all, are so designed that the carbons are brought together by means of springs or weights, and some form of 'brake' controlled by a system of electro-magnets checks themovement. As the carbons burn away the arc lengthens, the current weakens, the electro-magnets lose their grip, and the carbons move together until the increasing current puts on the brake again. Some of these lamps are 'semi-automatic' only, that is to say, the arc has to be struck by hand, while others perform this operation automatically as well, usually by an additional magnet which draws back the carbons by the correct amount after the arc is struck.
My frank advice to intending lanternists is to leave these lamps alone. Some of them are satisfactory up to a point, but they are all apt to be 'jumpy,' and on the whole the hand-fed type is in my opinion to be preferred.
Arc Lamps on Alternating Currents.—The alternating current is not so good as the continuous for lantern work with arc lamps: the light per ampère is not so great, the light has an irritating habit of travelling round the carbons and there is always a slight 'hum.'
The sum total of these drawbacks is nothing very serious, provided that proper arrangements are adopted, and I have frequently manipulated arc lamps on alternating circuits with such good results that professional lecturers have at first refused to believe that the circuit reallywasalternating.
As it is frequently stated that to obtain a steady light with an alternating current is impossible, I can understand their surprise, and I can also understand the statement in question, as the problem is usually tackled on entirely wrong lines.
It is almost always stated that arc lamps for alternating currents should be arranged with the carbonsvertical, and many makers actually so construct their lamps as to allow of this.
To obtain a steady light under these conditionsisimpossible and I pity anyone who attempts it; but the statement that this is the best method of working has been repeated so often that it seems to have been taken for granted.
The best arrangement (in my hands at any rate) is toslant the carbons as for the continuous current, and also to have the upper carbon cored and the lower one solid, but to use a rather larger lower carbon than would be correct if the main were continuous.
Also the upper carbon should not bequiteso far back as with D.C.; to have the front edges of the two carbons practically in line is about correct, but theexactposition should be carefully adjusted to obtain the steadiest light, and it will be found that a slight alteration makes a considerable difference.
It is also a great help to have a weak electro-magnet, or its equivalent, so arranged that it tends by its influence to keep the arc to the front. On some lamps this is provided for, as even with a continuous current it is quite harmless and, if anything, beneficial; but, if not, any competent mechanic can easily fit an 'Induction Ring,' consisting of a single turn of stout copper wire, which has sufficient magnetic influence to do all that is required (Fig. 32).
This ring must be wired in series with the arc itself, and as the current passing in it automatically reverses in synchronism with the arc, its effect isalwaysto deflect the arc in the same direction, and care must of course be taken that it is so wired that the deflection is forward and not backward. This is the exact arrangement I have myself adopted, and I never experience any difficulty on the score of the arc wandering.
Right-angled arc lamps, as described on pages52and53, are also very efficient on A.C. mains, and frequently these lamps are already equipped with electro-magnets for the purpose required. The 'hum' of an alternating current cannot be altogether eliminated, but can be reduced to a minimumby reducing the voltage as far as possible.
As has been already said, the A.C. lends itself readily to transformation of voltage, and I find in practice 90-100 tobe ideal. More than this is inclined to be noisy, and less is apt to result in an unsteady arc.
The arrangement, therefore, which I recommend from long experience is to employ a transformer to reduce the E.M.F. to 100 volts or thereabouts, and then work with a resistance in the usual way (if the original current is 100 volts, of coursenotransformer is required) with a properly constructed arc lamp fitted with an induction ring or electro-magnet. No difficulty should then be experienced in obtaining a good, steady, and fairly quiet light.
Fig. 32.--Arc Lamp with Induction RingFig. 32.—Arc Lamp with Induction Ring.
Any little 'hum' remaining can be silenced to a very considerable extent by placing the entire lantern on a thick block of saddlers' felt, but in practice I have never found this necessary with ordinary currents, though a few abnormal circuits where the 'periodicity' is very high are noisier than others.
Fig. 33.--The Optical System of a LanternFig. 33.—The Optical System of a Lantern.
The following table gives the sizes and particulars of carbons for various currents that I have found best in actual practice:
THE OPTICAL SYSTEM OF A LANTERN
Fig. 33a.--Optical System of LanternFig. 33a.—Optical System of Lantern.
As previously noted, the essential parts of an Optical Lantern are, in order from rear to front: (1) The illuminant; (2) the condenser; (3) the slide and slide stage; (4) the objective, to which must be added, (5) the body or framework which holds the whole together. Fig. 33 is a diagrammatic representation of the entire optical system and Fig. 33Ashows all the various partsin situ:Abeing the illuminant, shown in Fig. 33as an arc lamp,Bthe condenser,Cthe slide stage, andDthe objective. The foundation, so to speak, of the whole instrument is of course the slide, which, as made in this country, consists of a square of glass 3¼ inches diameter, the slide itself being somewhat less than this on account of the binding, &c.; in making calculations it is usually taken as a 3-inch circle. Slides are usually made by binding together with strips of paper or cloth two such squares, on one of which is the photographic film or painting forming the picture, the other being simply a plain cover glass placed over the slide surface to protect it, and between the two being placed a paper mask with an aperture of whatever size or shape is required, that of the aforesaid 3-inch circle being usually taken as the standard or normal dimension for this aperture.
The slide being illuminated by one of the various methods discussed in the previous chapters, is focussed on the screen by the objective, which must be selected according to the size of picture required and the distance between lantern and screen.
These points will be gone into later, and also details as to various types of objectives and their respective advantages; but it may be said here that a lantern objective consists usually of a combination of lenses of 2 inches or 2½ inches diameter mounted in a rackwork focussing system at a distance from the slide of 6 inches to 18 inches, according to the length of its 'focus.' As our slide is from 3 to 3¼ inches diameter, it is evident that all the light radiating from this cannot possibly get through the objective unless it isconvergedupon it, and to do this is the function of the condenser. The following two diagrams, Figs. 34 and 35, will make the matter clear.
Srepresents our glass slide of 3 inches clear diameter,Rthe radiant or illuminant, andLour objective, shown here for the sake of simplicity as a single lens.
The slide is well illuminated by the light emanating fromR, but it is obvious that the bulk of this light will never pass through the lens, and, in fact, only the very centre of the slide will under these circumstances appear upon the screen at all.
Fig. 34.--Optical System without CondenserFig. 34.—Optical System without Condenser.
Fig. 35.--Action of CondenserFig. 35.—Action of Condenser.
What is evidently wanted is toconvergethese outer rays, or in other words to bend them in so that they also pass through the objective, and this is the function of the condenser as illustrated in Fig. 35. The condenser is here represented also by a single lens, but in practice it also is invariably constructed of two or even three lenses, for both optical and mechanical reasons. It is evident from the above diagrams that the condenser must be somewhat larger in diameter than the slide itself, and condensers for ordinary lantern work are usually 4 inches to 4½ inches diameter. The former sizewill suffice if the condenser is placed very close to the slide, but it is often advisable to leave a little intervening space, especially if the illuminant is a powerful one, in order to allow any condensation of moisture readily to evaporate and escape. Hence lanterns for long range work (which involve, of course, good illumination) are usually made with condensers of 4½ inches diameter. Lantern condensers of to-day usually take one of the two forms shown in Fig. 36, but the exact curve must be left to the manufacturer, as the focus of the condenser must have a definite relation to that of the objective. Taking, however, the design ofE, the most common of all, the two lenses should not be exactly similar unless the objective is pretty short in focus, or, in other words, unless the distance of the illuminant on the one hand and that of the objective on the other are approximately equal. If the lantern is intended for long range work, that is equipped with a long focus objective, the front component of the condenser should also be constructed longer in focus (that is to say, with a shallower curve) than the rear one, and it is amazing how careless manufacturers are in this respect. If, as is often the case, the lantern is fitted with several objectives of different foci, it is usually necessary to supply alternative condensers also, or at least to supply an interchangeable front component.
Fig. 36.--Forms of CondensersFig. 36.—Forms of Condensers.
If the entire condenser is too long in focus, light is lost; if too short, it is impossible to obtain an even disc, as there is invariably a dark patch either in the centre or round the edges.
The mounting of the condenser also varies with different makers; but it must be remembered in any case that it getsextremely hot, especially the back component, and hence the glass must be mountedloosein its cell, otherwise there is great danger of it cracking. Also the space between the components should be well ventilated, in order to provide for the escape of moisture which usually at the start of a lantern exhibition is deposited upon the glass, and should be got rid of before the actual lecture commences.
Even with all care, the back component of a condenser will sometimes crack, though such an accident should be a rare occurrence; and hence a professional operator will usually provide himself with a spare lens, and the condenser should be so constructed that it can readily be changed, and with as little delay as possible.
Condenser lenses as made in this country are usually ground from the glass known as 'English Crown,' and comparatively rarely crack; but they are very slightly green in colour. French condensers, on the other hand, are whiter, but the glass is more brittle, and a fracture a more common occurrence. The French variety are (or were before the war) cheaper and generally met with in cheaper instruments. More expensive lanterns are usually fitted with English condensers, as the tinge of green is almost imperceptible, and the advantage as regards greater security pretty considerable.
The Slide Carrier and Slide Stage.—Taking still the optical system of the lantern in order from back to front, we now come to the slide, slide carrier, and slide stage. The slide itself has already been described, and the carrier is simply a mechanical contrivance, usually of wood, designed for the purpose of readily changing the pictures and which in its turn fits into the stage of the lantern. It may be asked why, if slides are now always made to a standard size, the slide carrier should not itself be built into the lantern and form the stage; but the answer is, in the first place, that slides of a different size,i.e.American or Continental,maybe met with,and also that there are various mechanical slides on the market—for example, chromotropes or scientific models, such for instance as are made to illustrate the movements of the planetary bodies—and these slides are permanently mounted in wooden frames which could not be put into a carrier. The commonest form of carrier is that known as the 'Double Sliding' pattern (Fig. 37), which consists of a frame with two apertures for the slide, and an outer frame through which this itself slides and which fits the stage of the lantern.
Fig. 37.--Double Sliding CarrierFig. 37.—Double Sliding Carrier.
This carrier, as will be seen, allows the next picture to be placed in position in the second aperture while the former one is being projected, and at a signal from the lecturer, the inner frame slides smoothly through the outer, and the slides are thereby changed. This carrier is simple, cheap, and quiet in its action; its one disadvantage is that each alternate slide has to be inserted from opposite sides of the lantern, and unless the operator is fairly tall this almost necessitates an assistant. Nevertheless, the carrier is the most popular of any, its other advantages, especially as regards price, being so great. It is usually constructed in such a way that the slide, as it moves out from the central position, automatically rises in its groove in order to facilitate quick removal.
Another pattern deservedly popular is that known as'Beard's Dissolving Carrier' and is shown in Fig. 38. In this ingenious carrier all the slides are inserted from the same side, the oncoming slide being pushedin frontof its predecessor, and being therefore somewhat out of focus it produces a blur on the screen.
The movement is performed by pushing in a projecting handle, and on withdrawing this the slide which is finished with comes with it, and the finish of the movement presses the new slide back until it is in its proper position and in focus.
Fig. 38.--Beard's Dissolving CarrierFig. 38.—Beard's Dissolving Carrier.
The entire action is simpler than it sounds, and the temporary blurring of the image on the screen during the process of changing is supposed to give somewhat the effect of 'Dissolving Views,' and hence the name 'Dissolving Carrier.'
This appliance is three times the price of the 'Double Sliding' pattern, but the fact that it is worked from one side only is a decided advantage, though on the other hand it is not (unless great care is used) quite so silent in its action as the 'Double Sliding' type.
A further modification of this carrier adapts it to take any of the recognised 'foreign' sizes of slides, so that if a few American ones, for instance, are met with among a collection of English manufacture, there is no need to change the carrier.
There are other varieties of carriers on the market which there is no need particularly to describe, such as, for example, carriers fitted with roller curtains to give the effect of a curtain rolling up, magazine carriers to hold twenty-four or more slides and exhibit them in rotation, and other patterns too numerous to mention. Of these the reader must be left to judge for himself, but, generally speaking,simplicityin a carrier is the most important point to be looked for, and complications, however ingenious, should be avoided.
Fig. 39.--Focussing Action of LensFig. 39.—Focussing Action of Lens.
The lantern stage must also receive consideration, but it will be better to discuss it as part of the mechanical construction of the lantern.
The Objectiveis really the most vital part of a lantern, as the definition of the picture almost entirely depends upon the excellence or otherwise of this lens. This will be obvious at once when it is realised that the objective has to project on to the distant screen a greatly magnified image of the comparatively small lantern slide, and the intending purchaser is strongly advised to economise almost anywhere rather than on this item.
The action of a lens in focussing the image is perhaps best explained by a simple diagram (Fig. 39), from which it will be seen that all the rays proceeding from any one point on the object are re-converged (when the lens is in focus) to a definite point on the image, and the perfection of the picture depends upon the lens performing this function accurately.
The imperfections are chiefly two, viz. those known as chromatic and spherical aberration respectively. Chromatic aberration simply means that all the colours composing the original beam of, say, white light are not equally refracted or converged, and therefore do not meet again at the same spot (the well-known prism or lustre effect), and reveals itself by coloured fringes round the edges of the various details in the picture.
Fig. 40.--Achromatic LensFig. 40.—Achromatic Lens.
By spherical aberration we mean that the light falling upon the centre of a lens is not brought to a focus at exactly the same spot as the marginal rays, and a general want of definition is the result, usually accompanied also by a want of 'flatness' in the image, that is to say the edges of the picture do not focus at the same time as the centre.
Chromatic aberration is easily cured by using an achromatic or compound lens made by cementing together two lenses of crown and flint glass respectively, as in Fig. 40.
It will be seen that the flint glass component by itself is aconcavelens and therefore neutralises in part, or in whole, the convex crown lens. Flint glass has both greater dispersive power and also greater refractive power than crown glass, but fortunately not to the samedegree; hence a compound lens made in this way and with curves carefully worked out may have its chromatic effect entirely neutralised while retaining very considerable refractive or 'focussing' power, and simple achromatic objectives of this type are quite inexpensive.
In lanterns intended for Science demonstration, as distinct from the mere projection of slides, lenses of this pattern are very frequently used, as they will project the latter when required reasonably well, and for the demonstration ofexperiments or of apparatus on the screen have advantages that need not be discussed here.
For very long focus lenses also they are sometimes employed, as the trouble from spherical aberration is much less apparent with lenses of long focus than with short, and the difference in expense is much more in the former case than in the latter. For short focus lenses, however, as used in moderate-sized halls, they are not good enough, and the type of lens almost universally employed is that known as the 'Petzval' combination (Fig. 41).
Fig. 41.--Petzval CombinationFig. 41.—Petzval Combination.
This lens really consists of two achromatic combinations, the pair at the front being cemented together, and that at the rear having an air space between. The combination is so designed that the spherical aberration of the one pair neutralises that of the other, and the result is or should be a lens corrected both for chromatic and spherical aberration.
These lenses, however, vary very much in the perfection of their results, and as they are at present usually imported in bulk from France, the customer does well to insist upon a demonstration of his own particular lantern before acceptance.
The magnifying power of a lens depends upon its 'focus' multiplied by its distance from the screen, and the focus in the case of a simple lens is easily determined by the familiar 'burning-glass' experiment, that is by focussing an image of the sun upon a piece of paper and measuring accuratelythe distance the lens must be away to produce the most concentrated spot.
In practice it is sufficiently accurate to focus a distant window, or other luminous object, upon the paper, any error obtained by this method being for ordinary purposes a negligible one.
With a compound lens, such as a 'Petzval' combination, this method does not hold good, as the optical centre of such a lens is not necessarily midway between its two components.
The actual focus can be got pretty approximately by focussing a window or other object as before and measuring the distance from one definite point (say the front edge of one of the lens cells) to the paper, then turning it round and taking a second measurement from thesamepoint, the mean between the two measurements giving the actual focus.
In practice the 'simple equivalent focus,' as it is termed, of a lantern lens is usually determined by measuring the magnification of the image thrown upon the screen, when, by knowing the original size of the slide (a 'standard' slide of 3 inches diameter is usually taken) and the distance between lantern and screen, we get the focus from the following very simple equation:
Diameter of picture on screen (in feet) / Diameter of slide (in inches) = Distance between lens and screen (in feet) / Focus of lens (in inches)
Diameter of picture on screen (in feet) / Diameter of slide (in inches) = Distance between lens and screen (in feet) / Focus of lens (in inches)
Diameter of picture on screen (in feet) / Diameter of slide (in inches) = Distance between lens and screen (in feet) / Focus of lens (in inches)
or perhaps more simply still:
{Distance between lens and screen (in feet) × Diameter of slide (in inches)} / Diameter of picture (in feet) = Focus of lens in inches;
{Distance between lens and screen (in feet) × Diameter of slide (in inches)} / Diameter of picture (in feet) = Focus of lens in inches;
{Distance between lens and screen (in feet) × Diameter of slide (in inches)} / Diameter of picture (in feet) = Focus of lens in inches;
or, if we know the focus of the lens but want to know how far from the screen we must go to produce a given-sized picture, the formula will be:
{Diameter of picture (in feet) × Focus of lens (in inches)} / Diameter of slide (in inches) = Distance required (in feet).
{Diameter of picture (in feet) × Focus of lens (in inches)} / Diameter of slide (in inches) = Distance required (in feet).
{Diameter of picture (in feet) × Focus of lens (in inches)} / Diameter of slide (in inches) = Distance required (in feet).
It is handy for the lanternist to remember that, dealing with a standard 3-inch slide, a 6-inch lens willalwaysgive a picture whose diameter isone-halfthe distance from lens to screen, a 12-inch lens half this again orone-quarter, and a 9-inch lens half-way between the two.
Bearing these simple figures in mind, the approximate distance can usually beguessedsufficiently near for the first trial, and then the lantern shifted a little nearer or the reverse as required.
The following table may, however, be useful, as showing readily the magnification produced at different distances by lenses of given foci:
The Diameter of the Objective.—The diameter of the objective must depend to a certain extent upon its focus in the case of a double combination such as a Petzval. These lenses consist, as has already been said, of two achromatic components some distance apart, and for technical considerations, which need not be discussed here, thedistancebetween these components is usually about two-thirds of the focal length. This is not a universal rule, as the lenses of different makers vary a good deal; but it is generally a factthat the longer the focus of the lens the greater is usually the separation between the two lens systems.
The entire lens therefore mounted in its tube resembles atunnelof varying length according to its focus, and through this tunnel aconeof light rays have to be passed. It is plain, therefore, that a lens of long focus, which in practice means a long tube length, must be made also of large diameter, or a portion of the cone will be cut off, with a consequent loss of light.
In practice lenses up to 6 inches focus are usually made of 2 inches diameter, and there is no advantage in a larger size. With a lens of 8 inches focus there is a slight gain in increasing the diameter to 2⅜ (the next 'standard' size), and lenses of longer focus than this should certainly be 2⅜ inches up to, say, 12 inches focus, when a lens of 3 inches diameter is preferable. These large lenses are, however, very expensive, both in themselves and also on account of the fact that their weight entails heavy and expensive brass mounting, and hence lenses up to 14 or 15 inches focus are often supplied in the 2⅜ size for reasons of economy.
To sum up,short-rangelanterns, as they are called, are usually fitted with lenses of 2 inches diameter, andlong-rangeinstruments either with 3-inch lenses or the intermediate size of 2⅜ inches. If a lantern is purchased for either long or short-range work, it is usually fitted with a brass front for a large lens, and so arranged that a shorter focus lens of 2 inches diameter can easily be interchanged, utilising the same brass mounting.
Lenses ofvariablefocus have also been designed, in which an additional lens can be added or subtracted to increase or decrease the focal length; but nothing very practical has yet been achieved in this direction, and therefore these 'Omnifocal' lenses have never come into general favour.
Objectives like condensers want cleaning at times, and care must be taken not to scratch the glass, as the concave lens of each component is of flint glass, and very soft. Aclean chamois leather is the best thing to use, but a soft cloth, or even a handkerchief, may be employed with care. It is very important that a lens be reassembled, after cleaning, the correct way, as a single lens reversed would utterly spoil the definition. The front component is usually balsamed together, and therefore all that is needed is to see that the whole combination is not reversed. In the Petzval system this lens should have its convex constituent towards the screen (Fig. 41). The back combination is usually loose, and the two lenses are sometimes separated by a thin brass ring. In the Petzval lens the concave element should be inside, with its concave surface outwards, the deep curve of the other lens should fit into this concavity, and the flatter curve face towards the condenser. One or two makers, however, have introduced a modification of the Petzval system in which the whole of this back combination is reversed, and the exact arrangement should therefore be noted very carefully when taking the lens to pieces.
THE BODY OF THE LANTERN
We now come to the mechanical construction of the optical lantern, and a great variety of design presents itself, according to price, type (i.e.short range or long range), and the individual ideas of the various makers.
Lantern bodies as a rule are now made of metal, although up till quite recently the better class instruments were more usually made of polished mahogany lined internally with iron; but there has of late been a consensus of opinion in favour of metal only.
In the cheaper lanterns this metal body is usually made either of Russian iron or of sheet-iron tinned and japanned,there being little to choose either in price or quality between the two varieties, and in all but the very cheapest instruments the front is usually of brass.
In better lanterns the body is more often made of enamelled steel, the front as before being of brass; but brass, copper, or aluminium are also used occasionally for the body of the lantern.