The Dichroscope.
If the observer looks through the lens (A) toward a bright light, as, for example, the sky, he apparently sees two square holes, [Fig. 4].
Fig. 3.
A, simple lens; B, piece of Iceland spar with glass prisms on ends to square them up; C, square hole.
Fig. 4.
What has happened is that the light passing through the square hole (C of [Fig. 3]) has divided in passing through the strongly doubly refracting Iceland spar (B of [Fig. 3]) and two images of the square hole are thus produced.
If now a stone that exhibits dichroism is held in front of the square hole and viewed toward the light, two images of the stone are seen, one due to its ordinary ray (which, as was said above, will have one color), and the other due to its extraordinary ray (which will have a different color or shade of color), thus the color of the two squares will be different.