O^ptical Curiosities of Literature. 



223 



ofiered to change the glass, a plano-convex was then and there 

 fitted on. Presently observing, to my surprise, that lq trying 

 something, he had turned the round side to the object, I asked if 

 he had any reason for preferring that side to the other. To this 

 he replied that — yes — it certainly would work with the other side 

 just as well ; either side would do ; there was no diflerence in that 

 way, only it must be a plano-convex. 



It was something of a shock to me to find that an optician of 

 no obscure name did not know the meaning of a plano-convex lens ; 

 but this was only a beginning. After some more incidents of the 

 same kind, I was destined to receive another shock very much more 

 severe. In conversation with the head of another house, I hap- 

 pened to remark on the smallness of the mirror in a microscope 

 that was standing near. The optician replied that — yes — it ivas 

 small, unusually small perhaps, but — it came to the same thing in the 

 end. The fact is, he proceeded to inform me confidentially, there 

 was a great misapprehension about the size of mirrors : few — very 

 few, were aware of a fact of which he had satisfied himself by 

 demonstration, that the illumination was exactly the same, whether 

 the mirror was smaller or larger. I expressed anxiety to know 

 something more of it ; and, calling for pen and paper, he drew for 

 me the following figure : — 



" Now," he said, " you see here is the large mirror A B sending 

 in a pencil of Hght to the point 0. Very well ; now suppose a 

 smaller mirror placed in this way, as X Y, you see how it comes 

 to just the same thing ; stands in exactly the same angle of light, 

 and 80 sends in the same pencil." 



With this the last remnants of my faith passed away, and I 

 ceased to be surprised at anything. I might add some more curio- 

 sities of literature from my experience to these two, which were the 

 first and the last ; but their interest would depend on the names of 

 the opticians concerned, which, for obvious reasons, could not be 

 given. Those concerned in these cases were names perfectly well 



VOL. VII. R 



