Hence one of the best methods of determining the magnifying power of a telescope to measure the diameter of the emergent pencil of rays, after the FIG.

On the other hand, if the emergent streams overlap and the common part be examined, it is found to have all the properties of common light.

In order to isolate a polarized pencil of rays with a rhomb of Iceland spar, it is necessary to have a crystal of such a thickness that the emergent streams are separated, so that one may be stopped by a screen.

In general a stream of plane-polarized light undergoes no change in traversing a plate of an uniaxal crystal in the direction of its axis, and when the emergent stream is analysed, the light, if originally white, is found to be colourless and to be extinguished when the polarizer and analyser are crossed.

telescope has been adjusted to focus upon a star, and to divide the diameter of the object-, glass by the diameter of the emergent pencil.