298 EDWAED M. NELSON ON THE FOCUS- APERTURE RATIO. 



objectives only the best lenses to have are the 1 in. and tbe 1/4 in., 

 it could be said that the best lenses were those of N.A. 0*2 and 0-68. 

 The advantage of this nomenclature becomes evident when we 

 consider that a 1 in. and 1/4 in. means one thing on an instrument 

 having a long tube, but quite another thing on one having a short 

 tube. Now, the conditions to be expressed are, that for observa- 

 tional purposes the best combined powers, with two eyepieces, are 

 50, 100, 200, and 400, with N.A. 0-2 and 0-68. If therefore the 

 lenses are defined by their numerical apertures instead of by their 

 foci, the subject is cleared of all ambiguity, whether the microscope 

 have a long or short tube. 



The delineation of an object depends on the N.A. of the 

 objective and not on the magnifying power, therefore this new 

 nomenclature would be quite rational. So we may say that with 

 two powers and two eyepieces the best objectives to have are 

 N.A. 0-2 and N.A. 0-68. 



Now, anyone can see that a long tube has an advantage, for if 

 it is given that for observational purposes the powers should be 

 50, 100, 200, and 400, these powers can only be obtained with a 

 short tube microscope by either a shorter focus lens or by a deeper 

 eyepiece, both of which are disadvantageous. 



As there are some hundreds of thousands of old eyepieces in use, 

 a table of the new powers allotted to them will be necessary, especially 

 as objectives are now engraved with their new multiplying powers. 

 Obviously if these new objective multiplying powers are used with 

 the old eyepiece multiplying powers, serious errors will arise. 



Numbers 12 3 4 5 6 



H . . 2 4 6 8 12 18 



K . .2-8 5-6 8-3 111 16-7 25 



A to E, Huyghenian. A, powers of Zeiss and most other Continental 

 makers. B, Leitz powers, the old and new are alike. C, exception 

 by Spencer. D, exception by Koristka. E, exception by Bausch and 

 Lomb. F, Winkel's Huyghenian and Complanats. G, Winkel's 

 compensating. H, Abbe's old powers, compensating, used by all 

 makers except Winkel. K, their new values. Example : The power 

 of an object-glass engraved 10, when used with an Abbe 18 eyepiece 

 is not 180, but 250, as shown in the above table. 



Journ. Quekett Microscopical Chib, Ser. 2, Vol. SIV., jVo. 88, November 1922. 



