

236 THE NORTHERN MICROSCOPIST. 

without going to the expense of two objectives of different apertures, 
but only yielding the same amplification. These are the apertures 
we consider more useful than those advised by Prof. Abbe, and we 
have given our reasons why. 
In our reply to the next question, Can apertures be reduced by 
means of a diaphragm without spoiling the performance of the 
objective? we must say that our experience is totally opposed to 
the statements of Prof. Abbe, and the echoes proceeding from 
the Editors of the Journal of the Royal Microscopical Society. If 
they had said some objectives will not bear this reduction, we could 
have said, Yea, verily! as the first objectives we tried to reduce in 
this manner gave extremely unsatisfactory results. Will Prof. Abbe, 
or any of his disciples in London, give us a diagram illustrating the 
distribution of the residuary aberrations in a one-inch objective, 
say of 35°, corrected, so that it will not break down under a } eye- 
piece, and show us Aow it must happen that cutting off the 
peripheral rays transforms a good into a relatively bad objective. 
M. Zeiss, in his catalogue, states most distinctly “the object 
glasses are constructed according to the formulas of Prof. Abbe, of 
Jena,” and we are to imagine that these lenses will not bear 
reduction ; those we have tried will not, and so Prof. Abbe is 
correct here ; but those of Tolles, Beck, Ross, and Wray, with which 
oe principally experimented, stand the reduction wonderfully 
we 
Perhaps our readers will say it is not enough to make a mere 
eg 7 have ap i we om and wish to ee ~~ 
pon the of photographs accompanying this num 
may be seen tap charies B and C ; the latter picture was taken with 
a specially constructed and well corrected one-inch objective of 
16° air angle with the A eyepiece, while the former (B) was taken 
with the Tolles inch of 35° cut down to 16° by means of the iris 
aperture shutter, with the A ocular, the amplification remaining the 
same in both cases, wis., 65 diameters. 
Our readers will now be able to judge whether cutting down the 
aperture of a good objective spoils it. For our own part we see but 
little difference in the quality of the two pictures, but where the 
difference is, that taken with Tolles’ objective has the superiority, 
the picture appeared more ‘solid’ with a sense of relief 
under the microscope, and this appearance been faithfully re- 
produced in the print. We have chosen Polycistina for our sub- 
ject, as a preparation of considerable depth ; diatoms would perhaps 
ve suited our purpose better as giving a picture more pleasing to 
the eye, but we wished specially to avoid thin flat preparations for 
of this demonstration. 
purposes 1 
_ Perhaps it would be impossible to reduce the aperture of a one- 
sixth of 178° air angle so that it might do duty as an objective of 
