218 Mr. T. E. Doubt on the Measurement of Colour 



upon its back face. It is about 5 millim. thick and some- 

 what prismatic, so that any reflexion from the front face is 

 thrown aside. Commencing about 7 millim. below the upper 

 edge a strip about 1 centim. wide is cut away, leaving the 



Figl. 



silvered surface with a nearly perfect mechanical edge. Over 

 this clear strip there is a microscopic slide silvered upon the 

 front face. The space between the mirrors is filled with a 

 medium the refractive index of which is intermediate between 

 that of the large glass and the microscopic slide. The distance 

 between the reflecting surfaces is not over a millimetre. 

 Fig. 2 shows the cone of rays HO projected upon a vertical 



