Dr. H. Dor on Vision in the Arthropoda, 233 
It is easy to calculate the focal distance, a b, of a lens, when 
we know the size of the object c d, its distance from the lens, a f, 
and the size of the image, gh. We get two triangles, c ad and 
hag, of which the sides and perpen- ¢ 
diculars are proportional. Thus we § 
geta:fa=hg:cd. Asthese threest--- 
quantities are known, it is perfectly 
easy to find the focal distance ab. a 
The author commenced by making numerous preparations, 
until he had the cornez of many insects in a sufficiently clean 
state to show the images very distinctly. He then pasted upon 
a window a rectangle of black paper of known dimensions, and 
measured accurately the distance from the stage of the micro- 
scope to the window. When the facets were in focus, they were 
seen very distinctly, but without images of the black paper; on 
their removal from the focus, their outlines became less clear ; 
but gradually small images of the black rectangle made their 
appearance, and grew more and more distinct, until by one or 
two turns of the screw they again disappeared. It was there- 
fore evident that each facet formed an image, and that this was 
at a sensible distance behind its posterior surface. It was there- 
fore a true lens, and the image must be reversed, which proved 
to be the case on examining another object. 
This observation proves that the image is not produced, as 
stated by Leydig, on the anterior surface of the crystalline body, 
acting as a bacillus, for the latter is often in immediate contact 
with the surface of the lens. Nevertheless, it might be urged 
that although the image is no doubt formed behind the optical 
centre of the lens, by the admission of even a thin layer of the 
so-called vitreous body the image might really be formed very 
close to the anterior surface of the crystalline body; or the cry- 
stalline body itself might be at once refractive and nervous, and 
the image might be formed at different depths in it according 
to the distance of the objects observed. To determine these 
points, the author entered upon a series of accurate measure- 
ments, of which the results are given in his paper. The follow- 
ing are those which bear most directly upon the subject :— 
OT oon enn rte 
Cornea of Musca vomitoria. 
Magnifying power 560 diameters. 
Size of the image seen under microscope=0'458. 
Real size of image=0-0082= +1, mill. 
Distance of object =68°8 centim. 
Size of the object =120 mill. 
Whence wv: ;453=688 : 120, from which e=,' mill. 
