Frazer. 



504 



[Feb. 20, 



In other words, the angle between the luminous ray and the glass plate 

 can never exceed 28° 09', or in round numbers 28 \ 



GM. Cover glass. 

 MN. Reflecting mirror. 

 OPR. Reflexion on object. 

 L'L'. Rays which pass through the objective. 

 D. Lens. 



T. Sliding tube carrying reflecting mirror. 

 Angle of incidence 62°. 



