490 PKOCEEDINGS OF THE SOCIETY. 



thickness — considering the cover, the front lens, and the intcr2)osing 

 medium as one.' " 



Mr. Stephenson being absent from town the following note from 

 him was read by Mr. Crisp. 



" I have read Mr. Wenham's note containing a reference to his 

 paper of 1870, of which, however, I was not aware when I brought 

 the subject of Homogeneous Immersion before the Society in 1878. 



" I do not understand what it is that Mr. Wenham claims. 



" The use of oil instead of water was suggested by Amici prior 

 to 1850 ; and it is equally clear that it was not until 1878 that any 

 homogeneous immersion objectives were produced in a practical form, 

 and then it was by Professor Abbe and Mr. Zeiss, and more recently 

 by Messrs. Powell and Lealand. 



" As, during a great part of the period between 1870 and 1878, 

 Mr. Wenham has been actively engaged in the construction of immer- 

 sion object-glasses, it is evident that he did not appreciate the 

 practical advantages likely to follow from the introduction of oil- 

 immersion glasses any more than Amici and other previous experi- 

 menters on the subject did. 



" This is not surprising when it is remembered that the very 

 essence of the homogeneous system depends under Professor Abbe's 

 able development on an optical princii:)le which Mr. Wenham has for 

 many years contended, and still contends, to be a physical impossi- 

 bility, viz. it gives an angle greatly in excess of even the ideal 

 maximum of a dry lens (180°).* 



" Moreover, if Mr. Wenham had attempted to give practical effect 

 to his suggestion of 1870 he would have found that identity of 

 refractive index between the cover-glass and immersion fluid was 

 by no means consistent with optical homogeneity, one of the most 

 essential conditions of which is identity of dispersion." 



Mr. Wenham said that it had not been his intention to raise any 

 controversy, but simply to record what he had done. If Amici had 

 given a distinct description of it he did not, of course, want to 

 claim it. 



Mr. Crisp said, that he thought it must be considered beyond dis- 

 pute that Amici was the first to suggest and to use oil as an immersion 

 fluid for objectives, in which he was followed by Oberhauser, Harting, 

 and others. They all ai>parently thought, however, that oil-immersion 

 objectives, were not capable of practical application. He read the 

 following extract from M. Kobin's book : — 



" We have seen that the principal obstacle to good resolution 

 arises from the violent refraction which the rays undergo on leaving 

 the cover-glass and passing into air, and again on their second 

 refraction by the front lens. Amici thought that to correct this 

 defect the front lens should form part of the cover-glass, but how 

 could the distance of the object from the lens be made variable ? 

 Simply by iuterj)osing between them an elastic medium having nearly 

 the same refractive index as the glass. He suggested that the lens 



* See 'M. M. J.,' v. pp. 16-17 and 118; vi. pp. 84-86 and 292 ; vii. p. 272 ; 

 xi. p. 113 ; xii. p. 222 ; xiii. p. 35, &c. 



