285 -^ /gS" '"€ X 



ON THE FOCUS-APERTURE RATIO. 



By Edward M. Nelson, F.R.M.S. 



{Read December 13th, 1921.) 



Figs. 1-5 in Text, 



Probably everyone will agree tliat this important point in the 

 economy of the microscope is in a deplorable condition. Micro- 

 scopists have a vague notion that if an objective has too much 

 aperture (N.A.) for its power, although, when a slotted condenser 

 is used, it will resolve a great number of lines to an inch, neverthe- 

 less, for ordinary microscopical work with axial illumination the 

 lens is not satisfactory ; chromatic and spherical aberrations are 

 sure to be in evidence. On the other hand, if an objective has too 

 small an aperture, the image given is similar to that seen in a 

 microscope purchased as a present for a schoolboy ! Obviously, 

 therefore, somewhere between these two limits there will be a 

 condition which is correct. But what is that condition ? It is 

 with a view to answering that question this paper has been 

 written. This subject has been previously dealt with upon three 

 occasions, about the same time, by Prof. Abbe, the Royal 

 Microscopical Society, and myself. 



The ratio of aperture to power (or what is the same thing, to 

 focus), as suggested by Prof. Abbe, leans very much towards the 

 schoolboy's microscope ; so much so that lenses with such a low 

 ratio of aperture to power have seldom been made, and it would 

 be difficult to obtain a specimen to see what they are like. In my 

 collection there is only one modern example ; it is a cheap 1 in. 

 formed by a single achromatic combination. It is well corrected, 

 but as it lacks the required N.A. it is never used. For other 

 examples it is necessary to examine object glasses made in 1840, 

 or earlier. When you are told that Prof. Abbe's ideal 1/4 in. has 

 an aperture of N.A. 041, and his 1/8 in. of N.A. 0-65, you will see 

 that we need not detain ourselves any longer over his list. 



The Royal Microscopical Society's official list seems to have 

 been drawn up at random, certainly not upon any kind of plan ; 



