

A PLEA FOR WIDE APERTURES. 237 

60°, but such differences we have never attempted and so cannot say 
whether they are possible or not, or even whether any one would 
be likely to attempt such reductions in actual practice; but we do 
know that our 2” of 20° will bear reducing to 12°, the one-inch of 
35° can be reduced to 16°, the half-inch of 66° to 35°, and the ¢inch 
of 120° to 70° by means of the aperture shutter with advantage 
whenever penetration is desired, though we are strongly of opinion 
that the method to be presently described is better for many pur- 
poses, especially when working distance is required to be increased. 
It only remains now to answer the third question :—Can “ pene- 
tration” and long working distance be secured from a series of 
objectives of wide aperture? The opponents to deep eyepiecing 
object to this operation as bad, resulting in bad definition, loss of 
light, and other evils. This shows us clearly that those experi- 
menters were either possessed of badly corrected objectives, or 
employed too-deep eyepieces to objectives of low angle, and no 
doubt the evils they complained of were apparent. We have, 
however, shown that the Tolles inch admitted more light when 
used with the C eyepiece than the Zeiss B with the A eyepiece, the 
amplifications being 128 (diameters) in each case, the objection 
that there is loss of light must therefore be abandoned. Prof. 
Abbe next tells us (NORTHERN Microscopist, p. 206, line 11), 
“ Forcing a high amplification from a low-power objective is always 
connected with a considerable loss of sharpness of definition.” We 
do not appear to be favoured with the exact definition of “ high 
amplification,” but may form an opinion as on the same page, line 
50, it is stated “ No experienced histologist will ever use in ordin- 
ary work even, an’ocular amplification of the amount necessary for 
obtaining 100 diameters from a one-inch objective or 200 from a 
half-inch.” To which we add, that no histologist ever will, except 
foolishly, so long as low powers of small aperture are in his cabinet, 
for the simple reason that he will lose both light and definition. 
That definition is not impaired by forcing 128 diameters from 
Tolles inch objective is proved by the photographs A and D. 
The slide was viewed with Ross and Co.’s A eyepiece, and the 
Zeiss B of 40°; Tolles inch was then substituted in combination 
with Ross C eyepiece, and the slide viewed again: when the best 
focus for the whole field was obtained, it was found that the largest 
circle upon the much broken Podocyrtis, near the centre, was 
exactly at the sharpest point, and a turn either way of the fine 
adjustment rendered it less sharp; photographs were then taken, 
the object being to secure perfect sharpness for this circle. 
The reader will see the results, they require no comment; even 
with the C eyepiece the picture yielded by the Tolles lens has 
better definition, more penetration, and a flatter field than the 
other picture. 
eC Ln ae 

