GRAY, ON A DOUBLY-REFLECTING PRISM. 



273 



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On the Form of a Doubltt-reflecting Prism, and its appli- 

 cation to the Microscope. By Peter Gray, F.R.A.S. 



Problem. 



A parallel pencil of light, of given magnitude, is incident 

 at right angles on one of the sides of a quadrilateral prism of 



glass ; entering the prism, it meets one of the adjacent sides. 



