150 Notice of a Chromatic Stereoscope. 
was drawn, lithographed, and cast off in three hours. The 
elegant work in 4to, entitled “ Portraits of the Leading Re- 
formers,” now in the course of publication by the inventors 
of the new process, shews the powerful effect produced by 
characteristic lithographs.— Edi¢.] 
Notice of a Chromatic Stereoscope. By Sir DAVID BREWSTER, 
K.H., F.R.S., V.P., R.S. Edin. Communicated by the 
Royal Scottish Society of Arts.* 
In the year 1848, I communicated to the British Associa- 
tion, at Swansea, a brief notice of the principle of this instru- 
ment. 
If we look with both eyes through a lens, about 23 inches 
in diameter or upwards, at an object having colours of dif- 
ferent refrangibilities, 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 the 
same degree of refrangibility, the ¢wo 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 refran- 
gible 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 de- 
viation 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 b/we one, in the 
very Same manner as the two nearest portions of the dissimi- 
lar stereoscopic figures stand up in relief at a distance from 
their more remote portions. The reverse of this will take 
* Read before the Society, Oth December 1849. 
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