152 
CORRESPONDENCE. 
perfectly at my house with the objective in question, angle 68°. I 
find the high opinion I expressed of Zeiss' glasses abundantly con- 
firmed by other observers; and Messrs. Powell and Lealand have just 
proved the correctness of Professor Abbe's view by making a ^th, or, 
more properly, a ith (for Mr. de Souza Guimaraens, and kindly lent 
to me by him for trial), which with a moderate angle reconciles in a 
remarkable manner the three demands for great working distance, 
penetration, and resolution, I find the angle, when the objective is 
corrected for pretty thick covering glass, about 105°. This glass, 
while fit for looking right into a macerated potato leaf for fungi, 
displays difficult diatoms very beautifully. I said that when our 
opticians were asked to do so, they would make small angles preserve 
their own special merits, and do what disproportionately large ones 
have hitherto been required for, 
I remain. Sir, your obedient servant, 
Henry J. Slack. 
Angle of Aperture, Chromatic and Spherical Aberration. 
To the Editor of the * Monthly Microscopical Journal.' 
1, Bedfoed Square, August 5, 1875. 
Sir, — I regret to find that Mr. Slack is " unable to understand some 
parts of my letter " on Chromatic and Spherical Aberration. If he 
will do me the favour to particularize the parts he does not under- 
stand, I will endeavour to make my meaning clear. Let me assure 
him, however, that the quotation he gives from Ganot's ' Physics ' 
simply confirms my statement, that spherical aberration is due to the 
forms of lenses ; and when he gave his own version of chromatic 
aberration I most assuredly dissented, and now leave it to those 
who are familiar with the subject to decide whether they will have 
Mr. Slack's definition of achromatism or that of well-recognized 
authorities on optics, who not only assert, but prove, that the condi- 
tions of achromatism depend only on the focal lengths of the compound 
lenses. 
With regard to Mr. Slack's statement that the more recently made 
objectives of Messrs. Powell and Lealand have more perfectly balanced 
corrections, this, I presume, is a well-deserved compliment to these 
opticians; but so far from its adding weight and strength to his 
assertion that " all chromatic aberration involves spherical aberration," 
it certainly proves quite the reverse. 
If proof were needed as to the value of large-angled glasses, I 
have but to refer Mr. Slack, and those who hold his views, to the 
Jurors' Eeport on the Microscope and its accessories, exhibited in the 
Great International Exhibition of 1851. He will there find a well- 
digested expression of the opinions of the highest authorities in favour 
of a new y%th, made by Messrs. Smith and Beck under Lister's direc- 
tion, angular aperture upwards of 75°, and which appears to have 
called forth the wonder and admiration not only of the J urors, but of 
