84 
Beck, on the Scales of lepidocyrtus. 
the markings are seen as at fig. 1 ; but in addition to this, 
and owing to the simplicity of the structure, the appearance 
fig. 2 produced by the least movement possible of the slow mo- 
tion will be exactly the same, either within or beyond the 
focus. On the other hand, if the object-glass be wrongly 
adjusted, which is the same as being badly corrected, there 
will be no distinct focus anywhere, the best being that shown 
at fig. 4 ; whilst on one side of the focus there will be strong 
lines (fig. 5), and on the other side a still more indistinct 
image (fig. 6) than that shown at fig. 2. This test supplies, 
therefore, to a remarkable degree, the means of detecting, to 
the greatest nicety, the exact point when a good object-glass 
is rightly adjusted; or, what is the same thing, the exact 
condition of the spherical aberration. A careful experiment 
with a one-eighth will show that, by moving the adjusting 
collar only one thirtieth of a revolution (a real movement 
between the lenses of less tlian ^Q^^yoth of an inch), there will 
be a marked difi'erence in the performance. Besides detect- 
ing the condition of the two aberrations, the Podura-scale 
will show an error in the centreing of the lenses by one side 
of each marking being darker than the other, or by a varia- 
tion of appearance when the direction of the scale in the field 
of view is changed. It is impossible to describe the effects 
produced by many other kinds of bad workmansliip, but gene- 
rally they will more or less resemble those of wrong correc- 
tion. It may be necessary to state here that the best kind 
of illumination is by an achromatic condenser of very small 
pencil, the light being either used direct or reflected by a 
L prism. In speaking of a test for the microscope, there is 
one other point to allude to, viz., aperture. The Podura-scale 
in itself is no positive test of aperture ; an increase of angle 
up to a certain point improves the brilliancy and sharpness 
of the markings ; but this particular matter may be put in 
the following form : — If the Podura-scale be shown well by 
an object-glass, measure its aperture by some mechanical 
means, and you can exactly predict how much it will do on 
Nobert^s lines, or on the Diatomaceous tests ; but reverse the 
proceeding, and you can by no means be certain that, from 
the mere fact of an object-glass separating fine lines or minute 
dots, it will also have a proportionate defining power over 
other and more general objects, which owe their appearance 
to an entirely different arrangement of structure. 
As regards the structure of the Podura-scales, my own 
observations were made without the knowledge, although they 
are entirely confirmatory, of what has already been published 
in the ^ Microgrnphic Dictionary.' It is stated there, under 
