234 Dr. H. Dor on Vision in the Arthropoda. 
This calculation leads to a very conclusive positive result. 
Leaving out the determination of the optical centre, the image 
was formed far behind the posterior face of the lens, the focal 
distance ;1, being exactly twice the thickness of the lens, which 
was 3/5 mill. 
The next point was to determine the length of the crystalline 
body, and to calculate at what part of it the image might be 
formed. The most favourable species for this purpose appeared 
to him to be the Macroglossus stellatarum, as its eyes are hard 
and prominent. The results are given from the mean of three 
measurements. 
Macroglossus stellatarum. 
First measurement.—Magnifying power 838 diam. 
Apparent size of the image 3:48, real size 0:0089= 1, mill., 
from which # (focal distance) = ', mill. 
Second measurement, the same eye, magn. 255 diam. ‘ 
_ Apparent size of the image 2°44, real size 0:0091= 4, mill., 
from which «= 5 = 7g mill. 
Thickness of lens=,', mill. 
Measurement of crystalline bodies (mean of five measurements 
of different crystalline bodies).—-Apparent size = 15°59, real size 
0:061= +; mill, 
The author was the more struck with this coincidence of the 
focal length of the lens and the length of the crystalline cone, 
as his only endeavour had been to prove whether the image was 
formed immediately behind the cornea, as was necessarily as- 
sumed in Leydig’s theory. The focal distance of the cornea 
measured in the air being = +, mill., and the summit of the 
cone being at the same distance from the posterior surface of 
the lens, the difference between the refraction of the air and that 
of the crystalline body is sufficient to shift the position of the 
image for a distance equal to that between the posterior surface 
of the lens and its optical centre. Thus, if, as stated by Zenker, 
the index of refraction of the crystalline body be 1°40, we should 
have, in the preceding case, 
1:00 : 0°0625 = 1:40: 2; 
from which = # = 0:0875 = circa }. 
Hence the optical centre would be placed forward in the 
cornea by the whole difference between 7; and ;4, or about 
zy mill. Now, as the antero-posterior diameter of the cornea 
in the Macroglossus is zz, the optical centre would be about 
ziz mill. behind the anterior surface of the cornea. 
The author concludes his paper with the following remarks; 
— We assume without hesitation that each facet is a complete 
