^ 



62 On Microscopes and Microscopy. 



* 



equate to the successful study of some points in Cytogenesis Snd 

 Spermatology. 



In regard to a comparison between the instruments of Ober- 

 hauser and Nachet, I cannot admit, as have some of my friends, 

 the great superiority of the latter over the former, that is, with 

 corresponding powers. Nachet works lenses having some 300 or 

 more diameters more than those of Oberhauser — ^besides with his 

 best, the angle of aperture is wider and of course the field larger. 



But still I have been unable to perceive tliat Nachet's lenses 

 have a whiter field, or define objects more sharply than those of 

 his tutor. I found many of the best microscopists and naturalists 

 in Paris, who affirmed that as yet they had not seen fit to give 

 up Oberhauser for Nachet. 



It now remains for me to say a word or two about Mr. Spencer. 

 He is now a rival at least of the best foreign artists, and from 

 what has been said, you cannot think it strange that they are 

 fast acknowledging him, as such. He has taken up the subject 



practically and experimentally, going over the whole ground for 



tiimself and learning its complete condition. It has been a source 

 of wonderment to foreign opticians, how he could obtain such 



wide angles of aperture with his lenses and yet having the gen- 



erally acconjpanying errors of chromatism and spherical aberra- 

 tion so carefully corrected; in fact Mr. Ross, in speaking of this 

 subject to me, said, he thought that Mr. Spencer must have some 

 mode of working glass as yet unknown to other opticians. 



I think it true that the best English opticians and especially 



Mr. Ross, have wrought the finest European glass to its utmost 



capacity. Now the secret of success with Mr. Spencer is, that 



I- he makes his own glass, and after much experimenting, he 



has been so fortunate as to produce a form capable of being 

 wrought with a very wide angle of aperture, yet with no more 

 than the ordinary liabilities to chromatism and spherical aberra- 

 tion. This is the great point to be sought, and in his lenses re- 

 xiently made, I think he has been quite successful, much more so 

 than with those made some three or four years since. 



The advantages of such a wide angle of aperture, with a cor- 

 rection of the above errors, cannot be appreciated except by use 

 of the lenses. The size of the field being very mucli increased, 

 one is able to perceive the whole object instead of a single por- 

 tion — and when you wish to view all the relations of an object 

 in conjunction, or where it is to be sketched — this is a qualifica- 

 tion of much value. Moreover the flood of light that the glass 

 K receives, emanating from every part of the object observed, ren- 

 tiers the latter very much more distinct, and its structure there- 

 ^Ibre can be much better made out. This is very noticeable and 

 its great advantage will be quickly perceived in^icroscopical pa- 

 thology, in the study of sections of tumors, &c., and in the deter- 

 mination of the structure of minute animals in tfatural history. 



a 



-**'[. " ' 



m 





