106 EXPERIMENTS, &c. 



what, without producing the evil which is said 

 to arise from the convexity of lenses ; and spec- 

 tacles of this kind might, with more propriety, 

 I think, than any others, be called presemers. 

 A like combination of such prisms, but with 

 their angles turned the other way, might, when 

 the object was moderately distant, be of service 

 to the short-sighted. But objects, very remote, 

 would be made by them to appear double. 





