Light entering the dichroscope is broken into two polarized rays that have vibrational directions at right angles to each other. The two images in the dichroscope represent the polarized light and the pleochroic colors will be represented in doubly refractive stones unless the stone is viewed thru the optic axis. Singly refractive stones will show only one color in the dichroscope.
The advantage of the dichroscope lies in the fact that the two pleochroic colors that may be characteristic of a given direction in a doubly-refractive gem are seen side by side. If no dichroism is detected in the first examination, the stone can be turned and viewed in other directions. Trichroic gems can also be identified and will show three colors if the stone is viewed in three directions perpendicular to each other.