MEMORANDA. 229 
ends pointing to it (B), the broad ends appear lke brilliant 
spots ; but when this direction is reversed (C), the light from 
the points is so slight that the scales appear to have lost their 
markings altogether. Now, if the object were an opaque 
substance, this result would be a convincing proof that the 
markings were depressions; but as we know it to be trans- 
parent, it follows that these particular appearances can only 
be produced by elevations.” 
Now, I cannot say that I see this sequitur at all; and I 
fancy that Mr. Beck’s argument is based upon false principles, 
inasmuch as he has ignored one of the principal conditions of 
all objects viewed under the compound microscope, viz., that 
their apparent position is reversed. In the case we are al- 
Iuding to I cannot help considering, with all due deference 
to such an eminent optician and microscopist as Mr. Beck, 
that the transparent Podura scale is rendered, under the cir- 
cumstances, practically opaque, and that in the example B 
the broad ends of the markings are really directed towards 
the left hand and source of light, and, being elevations, are, 
of course, illuminated; but as everything is reversed under 
the microscope, these illuminated ends appear on the right 
side, or that furthest from the light. Of course, the same 
argument applies to his cases A and C—the bright sides are 
really nearest the light, and are simply thrown over to the 
other side by the reversing power of the microscope. 
Again, in p. 63 it is stated that the magnifying power may 
be easily ascertained by comparing the magnified stage mi- 
cromatic lines traced by aid of the camera lucida with a rule 
divided into inches and tenths; but in such calculations par- 
ticular care must be taken that the distance from the edge of 
the camera lucida to the paper is exactly ten inches, as this 
is the standard distance of distinct vision with the naked eye. 
Though I am aware that this is a very generally accepted 
maxim, I fancy it may be an erroneous one, and that, to as- 
certain the exact magnifying power of any particular micro- 
scope, the paper should be the same distance from the prism 
as the eye-piece is from the micrometer on the stage, which 
will depend on the length of the tube, which varies consider- 
ably in different instruments. If I am wrong, I hope Mr. 
Beck will be kind enough to prove it to me, either practically 
or mathematically —F. H. Lane, President, Reading Micro- 
scopical Society. 
Angular Aperture.—In the report of the microscopes ex- 
hibited at the International Exhibition, 1862, from “the able 
