Laws of Reflexion from Crystallized Surfaces. 81 



very small, and therefore tan j3 or tan j3' is very great. But 

 this remark is of no value whatever in explaining the very sin- 

 gular phenomena which you have observed in the extreme case 

 just mentioned ; nor can I imagine any reason why there should 

 be a deviation, as there was in some of your experiments, when 

 the axis lies in the plane of incidence, since everything is then 

 alike on both sides of this plane. Indeed the whole of this 

 subject, which occupies the latter part of your Paper of 1819, is 

 very extraordinary and interesting ; and I was glad to hear that 

 you had resumed the investigation of it, and made many experi- 

 ments which have not been published. 



I wish you would publish them. They seem to be of great 

 importance in the present state of optical science. 



I am, dear Sir, ever truly yours, 



J. MAC CULLAGH. 



TBIN. COLL., DUBLIN, Dec. 22, 1835. 



