302 
COBRESPONDENCE . 
The answer to Mr. John Mayall, jun.'s, question, Whence conies 
the luminous field in the immersion lens if not from its having the 
power to collect rays which are totally reflected when the dry lens is 
used? is thus abundantly answered. It may come from the rays 
reflected from the lower end of the facet intentionally left to admit 
the light. 
I must decline to argue with Mr. Carr whether when the image 
under a ^yh breaks up by the test of deep oculars its amplifying 
power is " about 1000 diameters " only ? The answer is so obvious 
that the discussion is puerile : the initial power is greater. Equally 
unprofitable would it be to debate as to what Mr. Hogg did or did 
not say in his letter ; at pages 97 and 98 of your Journal it will be 
found at full length, with the " non sequitur " at which he has arrived. 
Your obedient servant, 
Crito. 
Chromatic and Spherical Aberration. 
To the Editor of the ' Monthly Microscopical Journal' 
1, Bedford Square, November 12, 1875. 
Sir, — Dr. Pigott's contribution on the question of chromatic and 
spherical aberration has at least fche demerit of introducing confusion 
into this subject. Would he have us believe he has discovered that 
everyone up to the present time, save himself, has held erroneous 
views on the well-recognized distinctions between these aberrations ? 
Dr. Pigott's attempt to persuade us that they " are identical in the 
nature of things," requires some explanation; for it appears to me 
he makes " confusion worse confounded," by calling two properties 
of lenses by the same name ; whether by so doing this will add to the 
clearness with which the subject is usually apprehended is a matter 
demanding further consideration. 
If, as he says, " all chromatic aberration involves spherical aber- 
ration," will he tell us how it happens that in the mathematical 
formula given by Herschel and other writers on optics, for the elimi- 
nation of chromatic aberration, the expressions for forms and order 
are not found ? And again, if these aberrations " are identical," why 
have they been discussed under separate and distinct propositions ? 
and why do practical opticians continue to treat them as separate and 
distinct matters, knowing well that the conditions essential to the 
correction of chromatic aberration do not necessarily involve those of 
spherical aberration, and vice versa ? They may, as Professor Parkin- 
son says, be corrected in one and the same combination ; nevertheless, 
the aberrations a,re in themselves perfectly distinct phenomena. 
I cannot well give Dr. Pigott a higher authority on this subject 
than the Astronomer Royal, whose words are, " The laws of spherical 
aberration and of chromatic aberration both in microscopes and tele- 
scopes are totally different'' 
I remain yours, &c., 
Jabez Hogg. 
