president's address. 203 



tion, a doublet may be very easily constructed with only 



— '214 — of aberration, a result 27 per cent, better than 



F. 

 anything hitherto realised in doublets of this form. 



There can be no doubt that Sir John Herschel's doublets 

 have played an important part in the evolution of the micro- 

 scope, as evidenced by the references made to them in almost 

 all microscopical text-books since their introduction in 1821 up 

 to recent times, say, for example, " Ency. Brit.," 9th edition, 

 art. " Microscope," p. 260. Although these doublets are now 

 of no use for visual purposes, never having equalled the per- 

 formance of a Brewster's grooved sphere, they are, and will 

 probably continue to be, of service for Bull's eyes and condens- 

 ing lenses. The so-called Herschel's doublets in the market 

 are not made on Herschel's formula, nor, in fact, on any 

 formula at all, but are for the most part composed of any two 

 lenses chosen at haphazard, and consequently have far more 

 aberration than two pianos of proper foci. In the following 

 Appendix there is 



1st. Sir John Herschel's doublet transposed to a combination 

 whose combined focus is 1*0. 



2nd. A doublet of no aberration of the same focus. The 

 second lens of this doublet is a bi-concave. You will observe 

 the very small radius of the incident surface of the crossed 

 lens. The arrangement has no practical value. 



3rd. My new doublet of the same focus. You will notice 

 that the deepest curve is the incident surface of the meniscus, 

 but it is not so deep as that of the corresponding curve in 

 Herschel's form. 



4th. The curves of the best form of microscope Bull's eye, 

 made of Boro- Silicate crown. It has 62° of aperture, and only 

 a total aberration of — '103 ; its working distance is 1*5. 



I understand that some other microscopist before me has 

 pointed out that Herschel's doublet was not free from aberra- 

 tion, but I am not aware if he proposed a better alternative 

 form. 



The authorities who have upheld the aplanatism of this 

 doublet so long before the microscopical world, have done so 

 because they have merely inspected the equations and have not 



