The chromatic stereoscope is a form of the instrument in which relief or apparent solidity is given to a single figure with different colours delineated upon a plane surface. If we look with both eyes through a lens LL, Fig. 42, about 2½ inches in diameter or upwards, at any object having colours of different degrees of refrangibility, such as the coloured boundary lines on a map, a red rose among green leaves and on a blue background, or any scarlet object whatever upon a violet ground, or in general any two simple colours not of the same degree of refrangibility, _the differently coloured parts of the object will appear at different distances from the observer_. [Illustration: FIG. 42.] Let us suppose the rays to be _red_ and _violet_, those which differ most in refrangibility. If the red rays radiate from the anterior focus R, or red rays of the lens LL, they will emerge parallel, and enter the eye at _m_; but the violet rays radiating from the same focus, being more refrangible, will emerge in a state of convergence, as shewn at _mv_, _nv_, the red rays being _mr_, _nr_. The part of the object, therefore, from which the red rays come, will appear nearer to the observer than the parts from which the violet rays come, and if there are other colours or rays of intermediate refrangibilities, they will appear to come from intermediate distances. If we place a small _red_ and _violet_ disc, like the smallest wafer, beside one another, so that the line joining their centres is perpendicular to the line joining the eyes, and suppose that rays from both enter the eyes with their optical axes parallel, it is obvious that the distance between the violet images on each retina will be _less_ than the distance between the _red_ images, and consequently the eyes will require to converge their axes to a _nearer_ point in order to unite the red images, than in order to unite the violet images. The red images will therefore appear at this nearer point of convergence, just as, in the lenticular stereoscope, the more distant pair of points in the dissimilar images appear when united nearer to the eye. By the two eyes alone, therefore, we obtain a certain, though a small degree of relief from colours. With the lens LL, however, the effect is greatly increased, and we have the _sum_ of the _two_ effects.