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DR. ROYSTON-PIGOTT. 



siderably in their working angular aperture, although this 

 experiment proves that it may rise to 107°. 



Theoretically, if it were possible to construct an object- 

 glass with nearly 180° angular aperture dry, the angle at which 

 the ray would enter from air into the substance of the front 

 glass is limited to about 41° 30' or 42°, or an angular aperture 

 of from 83° to 84°, according to the nature of the glass. 



