526 Transactions of the Society. 



We now come to the experiments which are of primary im- 

 portance to microscopists. No. 14 is an example of what can be 

 done with keen vision, the best lenses and apparatus, and expert 

 manipulation. Mr. Merlin has resolved the Amphiplcura pellucida 

 with a long tube apochromatic ^ of 0*98 N.A., and with a full 

 cone from an apochromatic condenser, using a Gifford's F line- 

 screen. His mean result with several objectives gives c a value 

 of 1-187, an angle of 5" -41, and 76,300 lines to an inch. This 

 last value and that in No. 14 are for white light ; they have been 

 deduced from results kindly given by Mr. Merlin, and obtained 

 by him with a Gifford's screen. These are exceptionally high 

 results, which cannot be approached by the ordinary microscopist, 

 with a good outfit and with fair experience. However, it will be 

 found quite easy, with a Gifford's F line-screen and an ordinary 

 paraffin lamp with a J-in. wick, to see Grayson's 25,000-band with 

 the long tube 24 mm., and the 55,000-band with the apochromat 

 12 mm. A Leitz short tube No. 6 will show the 60,000 very 

 strongly, and the No. 7 the 70,000 nearly as strongly. Eesults 

 such as these ought to be within the grasp of every microscopist 

 engaged upon serious work. 



The improvements in the optical portions of telescopes and 



microscopes consist in the increase of the -~ ratio. The 



focus 



earliest achromatics by John Dollond had apertures equal to 



/ f 



~; those by Thomas Cooke were £= ; the last Greenwich telescope 



• / • f f 



is — ; Zeiss apochromats are ~ to J —. As the denominators be- 

 -L-^ iZ 15 



come smaller, the difficulties opticians have to overcome enormously 



f 



increase. In camera lenses a modern anastigmat at J ~ yields 



8 

 -p 



sharper definition than an old rapid rectilinear did at {-. But 



16 



what are we to say of microscopical apertures 1 Any lens of 

 0-5 N.A. has an aperture of*!-, and similarly, a lens of N.A. 1-0 



has an aperture of —-. 



2 



In the face of these facts, it can hardly be expected that the 

 practical telescope limit, where c is a trifle less than unity, can be 

 maintained as a microscope limit. 



Passing on, an answer must be found for the important question, 



What are the actual telescope and microscope limits? Is the 



5" *55 

 Airy theoretical limit of to be accepted, or is the Dawes' 



