Proceedings of the British Association. 167 



and varying the quantity till he found the required proportion, he 

 obtained a mirror in platina, which appeared to have quite brill- 

 iant polish enough, and to be white enough to answer the pur- 

 pose ; and he considered, therefore, that Prof. W. had proved that 

 at least in one form the specula of telescopes might be made by 

 voltaic precipitation. His own idea was, that it might be possi- 

 ble to whiten the surface of the copper without injuring the 

 form ; and therefore, having obtained a speculum in very bright 

 polish, he (Mr. T) whitened it and transformed it into sulphuret 

 of copper : and after having retained it about a year, he did not 

 perceive the smallest alteration in any respect. This therefore 

 appeared to him a mode by which important results for astrono- 

 mers could be obtained. For the last year, perhaps, nothing fur- 

 ther had been done, either by Prof. Wheatstone or himself; but 

 the other day, being at Munich, he (Mr. T.) visited Prof. Stein- 

 heil, who showed him his inventions, and told him he had dis- 

 covered a method of making specula by the electrotype. It so 

 happened, that Prof. S. and himself had published their respec- 

 tive methods about a month or six weeks before : the Professor 

 having read a communication on the subject before the Academy 

 of Sciences at Munich, and printed it, and he (Mr. T.) having 

 published his in England. Their modes were, however, differ- 

 ent, as Prof. S. precipitated gold upon the speculum of copper ; 

 and having precipitated a certain thickness of gold, he then pre- 

 cipitated copper on the back of the gold, to give it sufficient 

 thickness. He (Mr. T.) should have thought beforehand that 

 gold would not reflect light enough to be available ; but Prof. S. 

 informed him he had found, by careful experiment, that it re- 

 flected more light than polished steel. He allowed Mr. T. to 

 look through a Gregorian reflecting telescope, of which the spec- 

 ulum was a common one but gilded, and he found, although a 

 slight tinge of yellow was thrown over all the objects, the image 

 was perfectly clear and well defined. Prof. S. said that in the 

 course of the year, he should have a very large telescope, fur- 

 nished not only with a speculum, but also with other apparatus, 

 voltaically formed, so that telescopes might all be made from a 

 good model, so as to insure greater accuracy of proportions : and 

 in this way even very large telescopes might be constructed at a 

 comparatively trifling expense. With reference to precipitating 

 copper on the back of the gold, the Professor had a simple expe- 



