CHAPTER XVII.ON CERTAIN DIFFICULTIES EXPERIENCEDIN THE USE OF THE STEREOSCOPE.

There are many persons who experience great difficulty in uniting the two pictures in the stereoscope, and consequently in seeing the relief produced by their union. If the eyes are not equal in focal length, that is, in the distance at which they see objects most distinctly; or if, from some defect in structure, they are not equally good, they will still see the stereoscopic relief, though the picture will be less vivid and distinct than if the eyes were in every respect equal and good. There are many persons, however, whose eyes are equal and perfect, but who are not able to unite the pictures in the stereoscope. This is the more remarkable, as children of four or five years of age see the stereoscopic effect when the eye-tubes are accommodated to the distance between their eyes. The difficulty experienced in uniting the binocular pictures is sometimes only temporary. On first looking into the instrument,twopictures are seen in place ofone; but by a little perseverance, and by drawing the eyes away from the eye-tubes, and still looking through them, the object is seen single and in perfect relief. After having ceased to use the instrument forsome time, the difficulty of uniting the pictures recurs, but, generally speaking, it will gradually disappear.

Fig. 52.

In those cases where it cannot be overcome by repeated trials, it must arise either from the distance between the lenses being greater or lessthan the distance between the eyes, or from some peculiarity in the power of converging the optical axes, which it is not easy to explain.

If the distance between the pupils of the two eyes,E, E′,Fig. 52, which has been already explained onFig. 18, islessthan the distance between the semi-lensesL, L′, then, instead of looking through the middle portionsno, n′o′, of the lenses, the observer will look through portions betweenoandL, ando′andL′, which have agreaterpower of refracting or displacing the pictures than the portionsno, n′o′, and therefore the pictures will betoo muchdisplaced, and will have so faroverpassedone another that the observer is not able tobring them backto their place of union, half-way between the two pictures in the slide.

If, on the other hand, the distance between the pupils of the observer’s eyes isgreaterthan the distance between the semi-lensesL, L′, then, instead of looking through the portionsno, n′o′of the lenses, the observer will look through portions betweennandL, andn′andL′, which have alesspower of refracting or displacing the pictures than the portionsno, n′o′, and therefore the pictures will be solittledisplaced as not to reach their place of union, and will stand at such a distance that the observer is not able tobring them upto their proper place, half-way between the two pictures in the slide.

Now, in both these cases ofoverandunder displacement, many persons have such a power over their optical axes, that by converging them to a pointnearerthan the picture, they would, in the first case,bring them backto their place of union, and by converging them to a pointmore remotethan the picture, would, in the second case,bring them upto their place ofunion; but others are very defective in this power of convergence, some having a facility of converging them beyond the pictures, and others between the pictures and the eye. This last, however, namely, that of near convergence, is by far the most common, especially among men; but it is of no avail, and the exercise of it is injurious when the under refracted pictures have not come up to their place of union. The power of remote convergence, which is very rare, and which would assist in bringing back the over refracted pictures to their place of union, is of no avail, and the exercise of it is injurious when the pictures have been too much displaced, and made to pass beyond their place of union.

When the stereoscope is perfectly adapted to the eyes of the observer, and thegeneralunion of the pictures effected, the remote parts of the picture, that is, the objects seen in the distance, may be under refracted, while those in the foreground are over refracted, so that while eyes which have the power of convergence beyond the picture, unite the more distant objects which are under refracted, they experience much difficulty in uniting those in the foreground which are over refracted. In like manner, eyes which have the power of near convergence will readily unite objects in the foreground which are over refracted, while they experience much difficulty in uniting objects in the distance which are under refracted. If the requisite power over the optical axes is not acquired by experience and perseverance, when the stereoscope is suited to the eyes of the observer, the only suggestion which we can make is to open the eyes wide, and expand the eyebrows, which we do in staring at an object, or in looking at a distant one,when we wish to converge the axes, as inFig. 22, to a pointbeyondthe pictures, and to contract the eyes and the eyebrows, which we do in too much light, in looking at anearobject, when we wish to converge the optic axes, as inFig. 21, to a pointbetweenthe pictures and the eye.

When the binocular pictures are taken at too great an angle, so as to produce a startling amount of relief, the distance between similar points in each picture, both in the distance and in the foreground, is much greater than it ought to be, and hence the difficulty of uniting the pictures is greatly increased, so that persons who would have experienced no difficulty in uniting them, had they been taken at the proper angle, will fail altogether in bringing them into stereoscopic relief.

In these observations, it is understood that the observer obtains distinct vision of the pictures in the stereoscope, either by the adjustment of the moveable eye-tubes, if they are moveable, as they ought to be, or by the aid of convex or concave glasses for both eyes, either in the form of spectacles, or separate lenses placed immediately above, or immediately below the semi-lenses in the eye-tubes. If the eyes have different focal lengths, which is not unfrequently the case, lenses differing in convexity or concavity should be employed to equalize them.

EDINBURGH: T. CONSTABLE,PRINTER TO HER MAJESTY.

Footnotes:[1]Edit. of Pena, pp. 17, 18, Paris, 1577; orOpera, by Gregory, pp. 619, 620. Oxon. 1703.[2]De Usu Partium Corporis Humani, edit. Lugduni, 1550, p. 593.[3]Joan. Baptistæ Portæ Neap.,De Refractione Optices parte, lib. v. p. 132, and lib. vi. pp. 143-5. Neap. 1593.[4]Trattata della Pictura, Scultura, ed Architettura.Milan, 1584.[5]Dr. Smith’sCompleat System of Opticks, vol. ii., Remarks, pp. 41 and 244.[6]Opticorum Libri Sex Philosophis juxta ac Mathematicis utiles.Folio. Antverpiæ, 1613.[7]InFig. 1,ahfis the optical pyramid seen by the eyea, andbgethe optical pyramid seen by the eyeb.[8]These angles are equal in this diagram and in the vision of a sphere, but they are inequal in other bodies.[9]Aguilonius,Opticorum, lib. ii. book xxxviii. pp. 140, 141.[10]It is obvious that a complete hemisphere is not seen with both eyes.[11]Aguilonius,Opticorum, lib. iv. pp. 306, 307.[12]In the last of these theorems Aguilonius describes and explains, we believe for the first time, theconversion of reliefin the vision of convex and concave surfaces. See Prop. xciv. p. 312.[13]Id., p. 313.[14]Opera, tom. ii. p. 394. Lugduni, 1658.[15]Opera Mathematica Optica, tribus libris exposita, p. 136.[16]Opticks, vol. ii., Remarks, pp. 41 and 245.[17]Id., vol. i. p. 48, Fig. 196.[18]Treatise on Optics, p. 171; see also sect. 64, p. 113.[19]Treatise on the Eye, vol i. p. 412, Plate 5, Fig. 37.[20]As Mr. Wheatstone himself describes the dissimilar pictures or drawings as “two different projections of the same object seen fromtwo points of sight, the distance between which is equal to the interval between the eyes of the observer,” it is inconceivable on what ground he could imagine himself to be the discoverer of so palpable and notorious a fact as that the pictures of a body seen by two eyes—two points of sight, must be dissimilar.[21]Phil. Trans., 1838, pp. 371-394.[22]Phil. Trans., 1838, pp. 391, 392.[23]December 28, 1550.[24]“Le fait est,” says the Abbé Moigno, “que le stéréoscope par réflexion était presque complètement oublié, lorsque Sir David Brewster construisit son stéréoscope par refraction que nous allons décrire.”—Cosmos, vol. i. p. 4, 1852.[25]Phil. Trans., 1852, p. 6.[26]Ibid., pp. 9, 10.[27]Vol. v. livre viii. p. 241.[28]Mr. Andrew Ross, the celebrated optician![29]The Abbé gave an abstract of this paper in the French journalLa Presse, December 28, 1850.[30]No. 54, Cheapside, and 313, Oxford Street. The prize of twenty guineas which they offered for the best short popular treatise on the Stereoscope, has been adjudged to Mr. Lonie, Teacher of Mathematics in the Madras Institution, St. Andrews. The second prize was given to the Rev. R. Graham, Abernyte, Perthshire.[31]Edinburgh Transactions, vol. xv. p. 349, 1843; orPhilosophical Magazine, vol. xxv. pp. 356, 439, May and June 1844.[32]Smith’sOpticks, vol. ii., Remarks, p. 107. Harris makes the difference ¹/₁₀th or ¹/₁₁th;Optics, p. 117.[33]This variation of the pupil is mentioned by Bacon.[34]Mr. Wheatstone himself says, “that it is somewhat difficult to render the two Daguerreotypes equally visible.”—Phil. Trans., 1852, p. 6.[35]A sheet of Queen’s heads may be advantageously used to accustom the eyes to the union of similar figures.[36]SeeEdin. Transactions, 1846, vol. xv. p. 663, andPhil. Mag., May 1847, vol. xxx. p. 305.[37]Bibl. Universelle, October 1855, p. 136.[38]Smith’sOpticks, vol. ii. p. 388, § 977.[39]Essay on Single Vision, &c., p. 44.[40]We may use also the lens prism, which I proposed many years ago in theEdinburgh Philosophical Journal.[41]See Chap. i. pp. 33-36.[42]For 1852, vol. xvii. p. 200.[43]These solid telescopes may be made achromatic by cementing concave lenses of flint glass upon each end, or of crown glass if they are made of flint glass.[44]Phil. Mag., Jan. 1852, vol. iii. p. 19.[45]American Journal of Science, 1852, vol. xv. p. 68.[46]See myTreatise on Optics, 2d edit., chap. vii. p. 65.[47]SeeCosmos, vol. ii. pp. 622, 624.[48]Id.vol. vii. p. 494.[49]Id.vol. iii. p. 658.[50]Phil. Trans., 1852, p. 7.[51]Mr. Wheatstone’s paper was published before I had pointed out the deformities produced by large lenses. See p. 130.[52]The Eye in Health and Disease, by Alfred Smee, 2d edit. 1854, pp. 85-95.[53]This expression has a different meaning in perspective. We understand it to mean here the point of the sitter or object, which is to be the centre of the picture.[54]Cosmos, Feb. 29, 1856, vol. viii. p. 202.[55]It is only in a horizontal direction that we can see 180° of the hemisphere. We would require a circle of eyes 2½ inches distant to see a complete hemisphere.[56]See ChaptersX. andXI.[57]When any external light falls upon the eye, its picture is reflected back from the metallic surface of the Daguerreotype, and a negative picture of the part of the Daguerreotype opposite each eye is mixed with the positive picture of the same part.[58]Modern Painters, vol. iii., Preface, pp. 11, 12.[59]Sir Francis Chantrey, the celebrated sculptor, shewed me, many years ago, a Sketch-Book, containing numerous drawings which he had made with theCamera Lucida, while travelling from London to Edinburgh by the Lakes. He pointed out to me the flatness, or rather lowness, of hills, which to his own eye appeared much higher, but which, notwithstanding, gave to him the idea of a greater elevation. In order to put this opinion to the test of experiment, I had drawings made by a skilful artist of the three Eildon hills opposite my residence on the Tweed, and was surprised to obtain, by comparing them with their true perspective outlines, a striking confirmation of the observation made by Sir Francis Chantrey.[60]By using large lenses, we may obtain the picture of an object within the picture of an opaque one in front of it; and with a telescope, we may see through opaque objects of a certain size. Many singular experiments may be made by taking photographs of solid objects, simple or compound, with lenses larger than the objects themselves.[61]In a landscape by Mr. Waller Paton, called the “Highland Stream,” now in the Edinburgh Exhibition, the foreground consists principally of a bed of water-worn stones, on the margin of a pool at the bottom of a waterfall. The stones are so exquisitely painted, that nature only could have furnished the originals. We may examine them at a few inches’ distance, and recognise forms and structures with which we have been long familiar. A water-ousel, peculiar to Scottish brooks and rivers, perched upon one of them, looks as anxiously around as if a schoolboy were about to avail himself of the missiles at his feet.[62]These views are well illustrated by the remarkable photographs of the Crimean war.[63]A French sculptor has actually modelled a statue from the stereoscopic relief of binocular pictures.[64]See my Treatise on the Kaleidoscope, second edition, just published.[65]“The importance of establishing apermanent Museum of Educationin this country, with the view ofintroducing improvements in the existing methods of instruction, and specially directing public attention in a practical manner to the question of National Education, has been of late generally recognised.”—Third Report of the Commissioners for the Exhibition of 1851, presented to both Houses of Parliament, p. 37. Lond., 1856.[66]This fine invention we owe to Mr. Paul Pretsch, late director of the Imperial Printing Office at Vienna. It is secured by patent, and is now in practical operation in Holloway Place, Islington.[67]An accomplished traveller, the Rev. Mr. Bridges, who ascended Mount Etna for the purpose of taking Talbotype drawings of its scenery, placed his camera on the edge of the crater to obtain a representation of it. No sooner was the camera fixed and the sensitive paper introduced, than an eruption took place, which forced Mr. Bridges to quit his camera in order to save his life. When the eruption closed, he returned to collect the fragments of his instrument, when, to his great surprise and delight, he found that his camera was not only uninjured, but contained a picture of the crater and its eruption.[68]A binocular slide, copied from the one originally designed by myself, forms No. 27 of the Series of white-lined diagrams upon a black ground executed in Paris. The drawings, however, are too large for the common stereoscope.[69]See Chap. i. p. 15.[70]Phil. Trans.1744.[71]Letterv. pp. 98-107. See also theEdinburgh Journal of Science, Jan. 1826, vol. iv. p. 99.[72]Journal, 1839, p, 189.[73]SeeEdinburgh Philosophical Journal, November 1832, vol. i. p. 334.

Footnotes:

[1]Edit. of Pena, pp. 17, 18, Paris, 1577; orOpera, by Gregory, pp. 619, 620. Oxon. 1703.

[2]De Usu Partium Corporis Humani, edit. Lugduni, 1550, p. 593.

[3]Joan. Baptistæ Portæ Neap.,De Refractione Optices parte, lib. v. p. 132, and lib. vi. pp. 143-5. Neap. 1593.

[4]Trattata della Pictura, Scultura, ed Architettura.Milan, 1584.

[5]Dr. Smith’sCompleat System of Opticks, vol. ii., Remarks, pp. 41 and 244.

[6]Opticorum Libri Sex Philosophis juxta ac Mathematicis utiles.Folio. Antverpiæ, 1613.

[7]InFig. 1,ahfis the optical pyramid seen by the eyea, andbgethe optical pyramid seen by the eyeb.

[8]These angles are equal in this diagram and in the vision of a sphere, but they are inequal in other bodies.

[9]Aguilonius,Opticorum, lib. ii. book xxxviii. pp. 140, 141.

[10]It is obvious that a complete hemisphere is not seen with both eyes.

[11]Aguilonius,Opticorum, lib. iv. pp. 306, 307.

[12]In the last of these theorems Aguilonius describes and explains, we believe for the first time, theconversion of reliefin the vision of convex and concave surfaces. See Prop. xciv. p. 312.

[13]Id., p. 313.

[14]Opera, tom. ii. p. 394. Lugduni, 1658.

[15]Opera Mathematica Optica, tribus libris exposita, p. 136.

[16]Opticks, vol. ii., Remarks, pp. 41 and 245.

[17]Id., vol. i. p. 48, Fig. 196.

[18]Treatise on Optics, p. 171; see also sect. 64, p. 113.

[19]Treatise on the Eye, vol i. p. 412, Plate 5, Fig. 37.

[20]As Mr. Wheatstone himself describes the dissimilar pictures or drawings as “two different projections of the same object seen fromtwo points of sight, the distance between which is equal to the interval between the eyes of the observer,” it is inconceivable on what ground he could imagine himself to be the discoverer of so palpable and notorious a fact as that the pictures of a body seen by two eyes—two points of sight, must be dissimilar.

[21]Phil. Trans., 1838, pp. 371-394.

[22]Phil. Trans., 1838, pp. 391, 392.

[23]December 28, 1550.

[24]“Le fait est,” says the Abbé Moigno, “que le stéréoscope par réflexion était presque complètement oublié, lorsque Sir David Brewster construisit son stéréoscope par refraction que nous allons décrire.”—Cosmos, vol. i. p. 4, 1852.

[25]Phil. Trans., 1852, p. 6.

[26]Ibid., pp. 9, 10.

[27]Vol. v. livre viii. p. 241.

[28]Mr. Andrew Ross, the celebrated optician!

[29]The Abbé gave an abstract of this paper in the French journalLa Presse, December 28, 1850.

[30]No. 54, Cheapside, and 313, Oxford Street. The prize of twenty guineas which they offered for the best short popular treatise on the Stereoscope, has been adjudged to Mr. Lonie, Teacher of Mathematics in the Madras Institution, St. Andrews. The second prize was given to the Rev. R. Graham, Abernyte, Perthshire.

[31]Edinburgh Transactions, vol. xv. p. 349, 1843; orPhilosophical Magazine, vol. xxv. pp. 356, 439, May and June 1844.

[32]Smith’sOpticks, vol. ii., Remarks, p. 107. Harris makes the difference ¹/₁₀th or ¹/₁₁th;Optics, p. 117.

[33]This variation of the pupil is mentioned by Bacon.

[34]Mr. Wheatstone himself says, “that it is somewhat difficult to render the two Daguerreotypes equally visible.”—Phil. Trans., 1852, p. 6.

[35]A sheet of Queen’s heads may be advantageously used to accustom the eyes to the union of similar figures.

[36]SeeEdin. Transactions, 1846, vol. xv. p. 663, andPhil. Mag., May 1847, vol. xxx. p. 305.

[37]Bibl. Universelle, October 1855, p. 136.

[38]Smith’sOpticks, vol. ii. p. 388, § 977.

[39]Essay on Single Vision, &c., p. 44.

[40]We may use also the lens prism, which I proposed many years ago in theEdinburgh Philosophical Journal.

[41]See Chap. i. pp. 33-36.

[42]For 1852, vol. xvii. p. 200.

[43]These solid telescopes may be made achromatic by cementing concave lenses of flint glass upon each end, or of crown glass if they are made of flint glass.

[44]Phil. Mag., Jan. 1852, vol. iii. p. 19.

[45]American Journal of Science, 1852, vol. xv. p. 68.

[46]See myTreatise on Optics, 2d edit., chap. vii. p. 65.

[47]SeeCosmos, vol. ii. pp. 622, 624.

[48]Id.vol. vii. p. 494.

[49]Id.vol. iii. p. 658.

[50]Phil. Trans., 1852, p. 7.

[51]Mr. Wheatstone’s paper was published before I had pointed out the deformities produced by large lenses. See p. 130.

[52]The Eye in Health and Disease, by Alfred Smee, 2d edit. 1854, pp. 85-95.

[53]This expression has a different meaning in perspective. We understand it to mean here the point of the sitter or object, which is to be the centre of the picture.

[54]Cosmos, Feb. 29, 1856, vol. viii. p. 202.

[55]It is only in a horizontal direction that we can see 180° of the hemisphere. We would require a circle of eyes 2½ inches distant to see a complete hemisphere.

[56]See ChaptersX. andXI.

[57]When any external light falls upon the eye, its picture is reflected back from the metallic surface of the Daguerreotype, and a negative picture of the part of the Daguerreotype opposite each eye is mixed with the positive picture of the same part.

[58]Modern Painters, vol. iii., Preface, pp. 11, 12.

[59]Sir Francis Chantrey, the celebrated sculptor, shewed me, many years ago, a Sketch-Book, containing numerous drawings which he had made with theCamera Lucida, while travelling from London to Edinburgh by the Lakes. He pointed out to me the flatness, or rather lowness, of hills, which to his own eye appeared much higher, but which, notwithstanding, gave to him the idea of a greater elevation. In order to put this opinion to the test of experiment, I had drawings made by a skilful artist of the three Eildon hills opposite my residence on the Tweed, and was surprised to obtain, by comparing them with their true perspective outlines, a striking confirmation of the observation made by Sir Francis Chantrey.

[60]By using large lenses, we may obtain the picture of an object within the picture of an opaque one in front of it; and with a telescope, we may see through opaque objects of a certain size. Many singular experiments may be made by taking photographs of solid objects, simple or compound, with lenses larger than the objects themselves.

[61]In a landscape by Mr. Waller Paton, called the “Highland Stream,” now in the Edinburgh Exhibition, the foreground consists principally of a bed of water-worn stones, on the margin of a pool at the bottom of a waterfall. The stones are so exquisitely painted, that nature only could have furnished the originals. We may examine them at a few inches’ distance, and recognise forms and structures with which we have been long familiar. A water-ousel, peculiar to Scottish brooks and rivers, perched upon one of them, looks as anxiously around as if a schoolboy were about to avail himself of the missiles at his feet.

[62]These views are well illustrated by the remarkable photographs of the Crimean war.

[63]A French sculptor has actually modelled a statue from the stereoscopic relief of binocular pictures.

[64]See my Treatise on the Kaleidoscope, second edition, just published.

[65]“The importance of establishing apermanent Museum of Educationin this country, with the view ofintroducing improvements in the existing methods of instruction, and specially directing public attention in a practical manner to the question of National Education, has been of late generally recognised.”—Third Report of the Commissioners for the Exhibition of 1851, presented to both Houses of Parliament, p. 37. Lond., 1856.

[66]This fine invention we owe to Mr. Paul Pretsch, late director of the Imperial Printing Office at Vienna. It is secured by patent, and is now in practical operation in Holloway Place, Islington.

[67]An accomplished traveller, the Rev. Mr. Bridges, who ascended Mount Etna for the purpose of taking Talbotype drawings of its scenery, placed his camera on the edge of the crater to obtain a representation of it. No sooner was the camera fixed and the sensitive paper introduced, than an eruption took place, which forced Mr. Bridges to quit his camera in order to save his life. When the eruption closed, he returned to collect the fragments of his instrument, when, to his great surprise and delight, he found that his camera was not only uninjured, but contained a picture of the crater and its eruption.

[68]A binocular slide, copied from the one originally designed by myself, forms No. 27 of the Series of white-lined diagrams upon a black ground executed in Paris. The drawings, however, are too large for the common stereoscope.

[69]See Chap. i. p. 15.

[70]Phil. Trans.1744.

[71]Letterv. pp. 98-107. See also theEdinburgh Journal of Science, Jan. 1826, vol. iv. p. 99.

[72]Journal, 1839, p, 189.

[73]SeeEdinburgh Philosophical Journal, November 1832, vol. i. p. 334.

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