242 CORRESPONDENCE. 



most microscopists about the classification of glasses. It so happens, 

 and it is a convenient arrangement, that they are named from their 

 focal lengths. The focus, therefore, has come to be considered the 

 essence of the glass, the aperture being counted only one of its 

 accidents or attributes ; like, for example, good or bad workmanship. 

 And so, for comparison of performance, we see objectives tried against 

 others of the same focus only. But there is, scientifically, no reason 

 whatever why glasses should not receive their rank or denomination 

 from theii' apertiu'es instead of their foci. Thus, for example, a glass 

 of 90° Avith half-inch focus may be compared with a glass of 90° 

 qixarter-inch focus with just as much fairness as against another half- 

 inch of 45°. Not only so, but it is even more reasonable, scientifically, 

 to compare by apertures than by foci, for the want of amplification 

 may be not ill compensated by the action of the eye-piece ; whereas by 

 no subsequent manipulation of the eye-piece can the peculiar qualities 

 depending on difference of angle ever be balanced. Focus and ajjerture 

 are in fact both essential factors in the denomination of an object- 

 glass, and where a difference exists in either we must keep in mind 

 that we are comparing diifereut things and not the same things with 

 differing qualities. A curious illustration of the confusion I have 

 commented on may be seen in a pamphlet or little book about micro- 

 scopes, rather widely cii'culated, I believe, some years ago — a reprint 

 from a paper read before the Eoyal Microscopical Society. The 

 author in a learned note vmdertakes to prove that objectives with wide 

 apertui-es are the best ; his reason being that with a half-inch of 90° 

 he can see things which he cannot see with a half-inch of 55°. It 

 does not seem to have occurred to him that, if this be all, an eighth 

 of an inch will show him more than either of them ; and therefore 

 with equal wisdom of logic he might have drawn the conclusion that 

 an eighth is preferable to a half-inch. 



It is customary in your Joiu-nal to sign with the writer's name ; 

 but as the purpose of this letter is only to ask a question, it will do 

 equally well, when asked, by a letter of the alphabet. 



Yours, &c., 

 B. 



To the Editor of the ' Monthly Microscopical Journal^ 



October 20, 1871. 

 Deab Sir, — Will you permit me through your columns to say, in 

 answer to inquiries as to where Dr. Sanderson's paper " On the Patho- 

 logy of Contagion " appears, and to which I refer in my observations 

 on the " Fungoid Origin of Disease," page 156, September number of 

 the Journal, that it is part of an Ajipendix to the Thirteenth Annual 

 Keport of the Medical Officer of the Privy Council, published by 

 Hansard, Great Queen Street, for the small sum of fourpence-half- 

 penny. 



Yours very faithfully, 



Dr. Lawson, Jabez Hogg. 



Editor • Microscopical Journal' ^c, ^c. 



