COREESPONDENCE. 
97 
Lastly, I think Mr. Slack might well spare himself the trouble of 
pleading ad misericordiam for those opticians whose specialty will be 
to " bring comparatively small-angled glasses to the highest degree of 
perfection in resolving as well as penetrating power." He may be 
well assured these opticians do not attach an exaggerated value to the 
" honour " of which he speaks as being within the power of the Koyal 
Microscopical Society to award to them ; what they ask for is a ready 
sale for their wares ; what they deprecate is, that one who occupies the 
position of Hon. Secretary of the Eoyal Microscopical Society should 
give such an extraordinary meed of praise to the productions of a 
foreign optician whose work cannot be said fairly to rival the highest 
class of work produced by the best English opticians. 
I am, Sir, your obedient servant, 
John Mayall, jun. 
Chromatic and Spherical Aberration. 
To the Editor of the ' Monthly Microscopical Journal.'' 
Bedford Square, July, 1875. 
Sir, — The great interest and importance to microscopists of the 
question discussed by Mr. Slack, " On Angle of Aperture," &c., in the 
June number of the Microscopical Journal, is my excuse for asking you 
to permit me to occupy a small portion of your valuable space on the 
subject. On reference to the paper it will be found that it contains 
statements which not only involve the use of the microscope, but also 
certain intricate problems on the construction of the achromatic 
instrument, which will appear to other of your readers, as they do to 
myself, quite at variance with standard authorities on optical or rather 
physical science. For instance, at p. 233, the following irreconcilable 
statements are made, which for the purpose of discussing I place in 
juxtaposition : 
" No one could deny that up to a 
certain date the best dot-displaying 
glasses had considerable chromatic 
errors, and that other glasses with 
better chromatic corrections did not 
show diflficult dots so well." 
"Hud it been considered that all 
chromatic aberration involves spherical 
aberration, the belief in any theoretical 
necessity for leaving considerable chro- 
matic error in order to ensure sharp 
definition would scarcely have become 
so prevalent." 
The first it will be seen involves a matter of fact, one which needs 
no such qualification as " that up to a certain date " it obtained, since 
it is perfectly true at this moment. In the next paragraph, however, 
Mr. Slack says, "all chromatic aberration involves spherical aberration." 
Now this is simply a matter of theory, and for which I can find no 
evidence or authority in any work on optics with which I am ac- 
quainted. In Parkinson's 'Optics' (p. 167) he will find it stated 
that " the conditions of achromatism depend only on the focal lengths 
of the compound lenses, not at all on their forms, or the order in which 
they are placed. By a suitable arrangement of these latter qualities 
(i. e. forms and order), the conditions requisite for the destruction of 
spherical aberration can be secured, and a compound object-glass 
