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IV. Notice of a Chromatic Stereoscope. 

 By Sir David Brewster, K.H., F.R.S., V.P.R.S. Edin* 



IN the year 1848, I communicated to the British Association, 

 at Swansea, a brief notice of the principle of this instru- 

 ment t- 



If we look with both eyes through a lens, about 2| inches in 

 diameter or upwards, at an object having colours of different re- 

 frangibilities, such as the coloured lines on a map, a red rose 

 among green leaves, or any scarlet object upon a blue ground, 

 or in general any two simple colours not of tbe same degree of 

 refrangibility, the two colours will appear at different distances 

 from the eye of the observer. 



In this experiment we are looking through the margin of two 

 semilenses or virtual prisms, by which the more refrangible rays 

 are more refracted than the less refrangible rays. The doubly- 

 coloured object is thus divided into two as it were, and the 

 distance between the two blue portions is as much greater than 

 the distance between the two red portions (red and blue being 

 supposed to be the colours) as twice the deviation produced by 

 the virtual prism, if we use a large lens or two semilenses, or 

 by the real prisms, if we use prisms. 



The images of different colours being thus separated, the eyes 

 unite them as in the stereoscope, and the red image takes its 

 place nearer the observer than the blue one, in the very same 

 manner as the two nearest portions of the dissimilar stereoscopic 

 figures stand up in relief at a distance from their more remote 

 portions. The reverse of this will take place if we use a concave 

 lens, or if we turn the refracting angles of the two prisms in- 

 wards. 



Hence it follows, and experiment confirms the inference, that 

 we give solidity and relief to plane figures by a suitable applica- 

 tion of colour to parts that are placed at different distances from 

 the eye. 



These effects are greatly increased by using lenses of highly- 

 dispersing flint glass, oil of cassia, and other fluids, and avoiding 

 the use of compound colours in the objects placed in the stereo- 

 scope. 



* Read before the Royal Scottish Society of Arts, Dec. 10, 1849. 

 t See Report of the British Association at Swansea, 1848, Trans, of Sect., 

 p. 48. 



