The Unimhlishcd Papers of J. J. Lister. 29 



subtended by the object-glass of the former and tlie large one in 

 the case of the Microscope, and how seriously coma interfered with 

 the action of the Microscope. 



" He then spoke of the improvements introduced by thee, and 

 certainly gave thee the full credit for the whole revolution in 

 microscopic excellence and observation, and said, moreover that 

 these improvements were the happiest instance of the application 

 of experiment and observation to the construction of the Micro- 

 scope ; also, that thy experiments had been most skilfully executed. 



" But I do not think he knew the main facts ascertained by thee ; 

 he said nothing about the aplanatic foci, or about over- or under- 

 correction, but merely said in substance as follows : — " Mr. Lister 

 observed that in some cases the coma turned towards the axis, in 

 others from the axis, and hence it occurred to him to make these 

 two effects counteract each other." Then, without givino- the why 

 or the wherefore at all, he said that the improved compound achro- 

 matic object-glass was first constructed thus — 



afterwards thus — 



" He then proceeded to say that experiment could never give 

 results so perfect but that theory might improve upon them ; that 

 the ^j/«7io-convex lens had been originally selected on account of 

 the facility of making the plane surface, and that theory shows that 

 lenses of some shapes have less amount of coma than others, and 

 hence by adopting other curv^es he thought (more \i\iQ\y fancied, I 

 imagine) there might be greater absence of coma attained to. 



" He concluded by merely expressing his idea that we had not 

 yet attained to the limits of improvement in the achromatic 

 Microscope, especially if larger instruments should come to be 

 used ; he also said that the reflecting Microscope requires atten- 

 tion, but gave no attempt at any explanation of the theory of the 

 achromatic object-glass further than I have mentioned — which was 

 rather a disappointment to me, and I fancy to others too. 



