ON THERMOPILES. 89 



day may come when we may not only get portraits and delineations 

 merely in monochrome, as it were, but get photographs of natural 

 objects in their natural colors. The review, which M. Tresca has 

 been so good as to give of the progress of French science, illustrated 

 by a selection of the apparatus used by philosophers of the great 

 names he has enumerated, is highly important. He has alluded 

 also to the difficulty he has experienced in collecting such objects, 

 simply because they are very seldom taken care of when once they 

 have served the purpose of their authors ; and has also mentioned 

 the grand institution in Paris, the Conservatoire des Arts et Metiers, 

 with which he is intimately connected, in which are contained some 

 of the most valuable records of the progress not only of science 

 but of the mechanical arts. He has also alluded to a rumour he has 

 heard that something of the kind is contemplated in England. I 

 only hope that this may have effect, and that in this country we 

 shall not in future have to regret the loss of apparatus which he 

 has deplored on the part of the French ; and that, eventually, all 

 those pieces of apparatus which have served and will serve for 

 scientific discovereries may be preserved for future generations. I 

 point again to this very beautiful and simple apparatus of Ampere's to 

 show what very small means are required to effect the most grand 

 discoveries ; and I ask you, again, to repeat your thanks to M. Tresca. 

 Mr. Ranyard had promised to give us a description of the astro- 

 nomical instruments contributed by the Astronomical Society, but I am 

 informed that they are not yet sufficiently well arranged for him to give 

 you his intended discourse, and we will therefore defer that to next 

 week ; and I will now call on Lord Rosse. 



ON THERMOPILES. 



The EARL OF ROSSE, D.C.L., F.R.S. : I have been asked to describe 

 in as few words as I can the thermopiles which I use with the moon, 

 as I believe they are more successful than any other apparatus which 

 have been tried. I am happy to do my best to make my method of 

 observation intelligible to the public. There are very few points I 

 need refer to. Of course if I went into the whole subject, and into 



