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for me,

or for Mr. Wheatstone, any refracting stereoscope :

“2, FEATHERSTONE BUILDINGS,

28th September 1854. “DEAR SIR,—In reply to yours of the 11th instant, I beg to state that I never supplied you with a stereoscope in which prisms were employed in place of plane mirrors. I have a perfect recollection of being called upon either by yourself or Professor Wheatstone, some fourteen years since, to make achromatized prisms for the above instrument. 1 also recollect that I did not proceed to manufacture them în consequence of the great bulk of an achromatized prism, with reference to their power of deviating a ray of light, and at that period glass sufficiently free from striæ could not readily be obtained, and was consequently very highpriced. - I remain, &c. &c.

“ ANDREW Ross. “ To Sir David Brewster.”

Upon the receipt of this letter I transmitted a copy of it to the Abbé Moigno, to shew him how he had been misled into the statement, “ that Mr. Wheatstone had caused a stereoscope with prisms to be constructed for me ;" but neither he nor Mr. Wheatstone have felt it their duty to withdraw that erroneous statement.

In reference to the comments of the Abbé Moigno, it is necessary to state, that when he wrote them he had in his possession my printed description of the single prism, and other stereoscopes, in which I mention my belief, now proved to be erroneous, that Mr. Wheatstone had used achromatic prisms, so that he had, on my express authority, the information which surprised him in my letter. The Abbé also must bear the responsibility of a glaring misinterpretation of my letter of 1838.

i The Abbé gave an abstract of this paper in the French journal La Presse, December 28, 1850.

In that letter I say that Mr. Wheatstone promised to order certain things from Mr. Ross, and the Abbé declares, contrary to the express terms of the letter, as well as to fact, that these things were actually constructed for me. The letter, on the contrary, does not even state that Mr. Wheatstone complied with my request, and it does not even appear from it that the reflecting stereoscope was made for me by Mr. Ross.

Such is a brief history of the lenticular stereoscope, of its introduction into Paris and London, and of its application to portraiture and sculpture. It is now in general use over the whole world, and it has been estimated that upwards of half a million of these instruments have been sold. A Stereoscope Company has been established in London2 for the manufacture and sale of the lenticular stereoscope, and for the production of binocular pictures for educational and other purposes. Photographers are now employed in every part of the globe in taking binocular pictures for the instrument, —among the ruins of Pompeii and Herculaneum— on the glaciers and in the valleys of Switzerland—among the public monuments in the Old and the New Worldamid the shipping of our commercial harbours—in the museums of ancient and modern life in the sacred precincts

a

1 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.

of the domestic circle-and among those scenes of the picturesque and the sublime which are so affectionately associated with the recollection of our early days, and amid which, even at the close of life, we renew, with loftier sentiments and nobler aspirations, the youth of our being, which, in the worlds of the future, is to be the commencement of a longer and a happier existence.

CHAPTER II.

ON MONOCULAR VISION, OR VISION WITH ONE EYE.

In order to understand the theory and construction of the stereoscope we must be acquainted with the general structure of the eye, with the mode in which the images of visible objects are formed within it, and with the laws of vision by means of which we see those objects in the position which they occupy, that is, in the direction and at the distance at which they exist.

Every visible object radiates, or throws out in all directions, particles or rays of light, by means of which we see them either directly by the images formed in the eye, or indirectly by looking at images of them formed by their passing through a small hole, or through a lens placed in a dark room or camera, at the end of which is a piece of paper or ground glass to receive the image.

In order to understand this let u be a very small pinhole in a shutter or camera, Mn, and let RYB be any object of different colours, the upper part, R, being red, the middle, y, yellow, and the lower part, B, blue. If a sheet of white paper, br, is placed behind the hole i, at the same distance as the object RB is before it, an image, br, will be formed of the same ray and the same colours as the object RB. As the particles or rays of light move in straight lines, a red ray from the middle part of R will pass through the hole - and illuminate the point with red light. In like manner, rays

om the middle points of

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Y and B will pass through and illuminate with yellow and blue light the points y and b. Every other point of the coloured spaces, R, Y, and B, will, in the same manner, paint itself, as it were, on the paper, and produce a coloured image, byr, exactly the same in form and colour as the object RYB. If the hole h is sufficiently small no ray from any one point of the object will interfere with or mix with

any
other

ray
that falls

upon
the
paper.

If the paper is held at half the distance, at b'y for example, a coloured image, b'y'r, of half the size, will be formed, and if we hold it at twice the distance, at 6"po" for example, a coloured image, boy"n", of twice the size, will be painted on

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the paper.

As the hole is supposed to be so small as to receive only one ray from every point of the object, the images of the object, viz., br, b'r', 6"/", will be very faint. By widening

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