NOTICES OF MEETINGS. 287 

employed for the general purposes of scientific investigation, that we cannot 
consider its deficiency to be compensated by the possession of any degree of 
resolving power whose use is comparatively limited.” It is to satisfy this de- 
mand for penetration, and to disarm their opponents, and reconcile them to the 
use of wide apertures, that the use of the aperture shutter is recommended. 
We must here = have recourse to our diagram, when we shall see upon 
what principle this instrument works. Supposing we have an aperture of say 
130°, we shall find that by cutting off the marginal rays that we leave an inner 
cone of say 65°. This of course will give increased penetration, but it is essen- 
tial to recollect that ‘‘the defining power of an objective depends upon the 
completeness of its corrections ;” that is, — the proper distribution of all the 
rays which pass through the objective. the use of a stop we sacrifice the 
completeness of the corrections by cutting off a large marginal zone of rays, and 
it scarcely needs demonstrating that an inferior image must de the result. Ina 
properly constructed objective the rays are scientifically distributed and utilized, 
and as a result, the focal point being further removed, we get a lower aperture 
without that serious sacrifice of marginal rays and loss of light necessary to good 
definition. We are here, however, met by an artifice which may be said to be 
‘*an expedient which minimizes the apparent difference in the performance of 
two lenses, one supposed to be ground to a low aperture, and the other reduced 
by a stop. This artifice, ere long, will be known as the photographic trick.” 
Before proceeding further, however, I would remind you that wide angled 
objectives are close workers, and the use of a stop does not increase the working 
distance. This circumstance alone militates against the use of the shutter. 
The difficulty in the way of illuminating and working with opaque objects with 
such powers as the 4-inch or 4-1oths objectives of high angles, combined with 
the serious loss of light in the use of a stop are sufficient for their rejection. In 
this month’s Northern Microscopist, page 238, occurs this passage :—‘‘ There 
are many lenses of this power (4-inch) in use possessing an og of 40°, used 
ially for the study of Foraminifera, Polycistina, and such like solid objects. 
ow, we argue that the half-inch of 40° is entirely unnecessary; a better image 
with more penetration, more light, and better definition may be secured by 
using the one-inch of 35°, and the C eyepiece, and thus giving considerably 
more working distance than could ever be obtained from a half-inch.” This 
opinion I am disposed to endorse. I should say that an inch objective of 25° 
for Polycistina, and a two-inch of 12° for Foraminifera, with the B eyepiece in 
each case, would give a better result. I will, however, take the paragraph as it 
reads, and I claim it as an important admission. It is a very unfortunate one 
for the writer, for it practically admits all I contend for, namely:—That a 
better image, of the same amplification, with better definition, with more 
working distance and more penetration, can be obtained with an aperture of 35° 
than one of 40, even under the forced amplification of a deep ocular. Now for 
the photographic test. I am sorry to say that my acquaintance with photo- 
phy is not sufficient to enable me to deal with this part of the subject as I 
ao ‘like. As, however, I cannot accept the photographic test, you will 
expect something like a reason. Without for a moment insinuating that they 
—the photographs—are not honestly and fairly produced, I should be better 
satisfied were I present when these photographs were taken. But I have another 
reason. I believe there are conditions set up and at work in the production of 
a photograph, which are not identical with the ocular use of the objective. Let 
me venture to explain. In addition to what is known as the chromatic spec- 
trum, coloured luminous rays, the result of decomposed light passing through a 
prism, there are a number of rays which neither give out light nor heat. These 
rays are called actinic or chemical rays. These rays, be it remembered, are 
non-luminous, invisible; they are of immense importance, however, to the * 
animal, vegetable, and mineral kingdom, and in promoting chemical changes. 
If, for instance, you put hydrogen and chlorine gases into the same bottle, and 
cause them to combine or mix chemically, they will explode with a loud report. 

