THE FOCUS-APEETURE RATIO. 297 



of course the same as before ; but this metbod reveals the correct 

 optical principles upon which it is based, while tbe other is a mere 

 juggling with figures. 



In our trade catalogues the Zeiss, or inferior, method has been 

 selected, and it usually appears somewhat after this fashion : 

 Objective, 40 mm. focus, code word " Jabber," initial power 6-3. 

 Eyepiece, 40 mm. focus, code word " Wock," initial power 3'7. 

 Combined power 23. Here we have two lenses of the same foci 

 with their initial powers depending upon whether their code names 

 are " Jabber " or " Wock ! " ^ The initial power of a lens is 

 10 divided by its focus in inches (that is, 10 times the reciprocal 

 of its focus), and this bears no relation to its code name, however 

 facetiously chosen. 



It is to be hoped that the members of this Club will refrain from 

 substituting " adjusted " for " initial " magnifying power, and 

 will designate the initial powers of their eyepieces correctly, and 

 put the allowance for tube length in the place where it should be, 

 that is on the objective.^ 



The old English system was a very good one : it consisted in 

 giving fictitious names to the lower-power object glasses. Thus 

 a true 2 in. was called a 3 in., a 1| in. a 2 in., and so on up to the 

 inch which was really 0-9 in. So one would take the initial magni- 

 fying power of their 2 in. as 5 whereas it was really a 1| in. with an 

 initial power of 6*7. Powers higher than 1 in. could be used 

 without artifice, but our manufacturers nevertheless gave them 

 fictitious values to enable them to palm ofi lenses with low 

 optical index upon unsuspecting purchasers. This procedure 

 has had much influence in driving the microscope trade out of the 

 country. A student, for example, buys an objective and thinks 

 he has got a 1/4 in. of N.A. 0-7, the optical index of which is 17-5, 

 but the lens that he does get is a 1/5 in. of N.A. 0-65, the optical 

 index of which is 13, a very difierent thing. The Germans, on the 

 other hand, have adhered pretty closely to their catalogue values, 

 and by so doing have secured a good market for their objectives. 



If schemes of the ratios of aperture to power, such as suggested 

 in this paper in curves (figs. 3, 4 and 5), are adopted, then it follows 

 that initial magnifying power and N.A. will practically become 

 synonymous terms. So, instead of saying that with an outfit of two 



^ Beginners shotild know that a Zeiss compensating Continental 

 eyepiece X Sis a great deal higher in power than the new Telaugic X 10. 

 With the same tube length and same objective, the 8-power compen- 

 sating gave a combined power of 188, while the Telaugic gave one of 160. 

 No textbook gives the slightest warning or even information upon this 

 microscopical trap. 



2 Since this was written, catalogues have been issued, in which this 

 nomenclature has been adopted. 



