"jouSi, aSTiS^I ctnd the Structure of the Podura Scale. 81 
reflected on the object under examination. Virtually, the point 
or source of light is in the principal focus of the prism, and the 
reflected rays are of course parallel. We thus obtain great intensity 
of illumination, which may be useful in the examination of certain 
structures, and the Httle vivid disc of light is easily thrown into the 
centre or on any part of the circumference of the field ; but hitherto 
the plane prism has answered every requirement in the examinations 
I have made, and of these one of recent interest has been the scale 
of the Podura. 
In the interpretation of this standard test object — the Podura 
scale — the value of parallel light from one source of light only will, 
I think, be admitted by all observers. But those who are about to 
use it must expect to see what they have never seen before ; for I 
can truly say with Mr. Wenham, whose results on a dark ground 
are a very close approximation to my own results on a light ground, 
that " this appearance is so diflerent from anything before seen in 
the Podura, that were I to exhibit it as such, not one of its nume- 
rous friends would recognize it." It is no slight satisfaction to 
feel that the support of so high an authority as Mr. Wenham will 
tend to make a priori objectors cautious. Mr. Wenham's paper is 
published in the ' Monthly Microscopical Journal ' for July. 
The following description is accepted by friends who have 
worked with me. The scale of the Podura consists of two mem- 
branes, between which there is a series of small solid spherules. 
These spherules or beads are often arranged in parallel rows 
towards the edge of the scale, and in the centre they are placed 
rather diamond- wise. Under a power of 12,000 linear, I have found 
24 spherules in -oVotti of an inch on the 12-inch horizontal dia- 
meter of the field and 6 on the vertical diameter. Hence, in the 
latter direction they are about e^oVo th of an inch apart, and in the 
former, the interval being equal to a diameter of a spherule, they 
are about 4^io o^h of an inch apart. If now we could place a 
series of spherules in almost close contact on the vertical diameter, 
we should have parallel rows of about 48 spherules enclosed between 
the membranes as in a tube, and the membranes themselves would 
touch and be in close contact along the parallel intervals. Now let 
this close contact of the membranes continue, since in point of fact 
it really does exist on the scale, but remove the spherules we have 
supposed to be inserted. Then we have an empty space like the 
empty finger of a glove between spherule and spherule on the 
vertical diameter of the field. The sides of this tubular space can- 
not preserve their parallelism without the support of the supposed 
additional spherules, and therefore they tend to fall together, having 
the diameter of the existing spherule for the width of the tube close 
to the spherule, and thence tapering to a point just before a lower 
spherule is reached. Thus we have on the vertical 12-inch dia- 
VOL. II. G 
