THE THEORY OF ARTHROPOD VISION. 561 
It further corresponds with the layer which I believe to be the 
sentient retina (see Pl. XL., Fig. x). 
The Erect Image in the Eye of Lampyris.—Although Exner 
rejects my view of the position of the retina, he gives a figure 
of a section of the eye of Lampyris splendidula [252 Taf. r. 
Fig. 1] in which he represents the dioptron and retina exactly 
as I do, only he terms the dioptron a vitreous. 
Exner further found an upright image of external objects 
immediately beneath my membrana basilaris, and he not 
only saw, but photographed this image. It is reproduced in 
his book [252, frontispiece], but in its printed form it is re- 
versed; the reversal, however, is due, as he explains, to the 
method of photo-printing employed. 
Exner's Theory of Super-position of Images.—Exner supposes 
that all the rays which fall from a point upon all the lenses of the 
compound eye form, after refraction, the envelope of a caustic 
curve, the cusp of which forms a point in the image and lies 
upon the retina. He regards his so-called vitreous as a perfectly 
transparent medium in which the caustic surfaces formed by 
the intersection of the refracted rays lie. This theory might 
be conceivable if such a vitreous existed in the compound eye, 
but, unfortunately for the theory, no such vitreous exists in any 
insect I have examined; if it exist in Lampyris spendidula 
the compound eye of this insect would be utterly unlike that 
of any other Arthropod; and there are indications in Exner’s 
figure that his so-called vitreous differs in no marked manner 
from the great rods, as they appear in the recent eye, with the 
pigment but little developed. 
With regard to his mathematical demonstration, he assumes 
that the surface of the cornea is a segment of a sphere. Now, 
so far as my observations go, it never is a segment of a sphere. 
He further assumes that the light-rays leave the cone as a 
parallel-rayed pencil, and his circles of confusion have approxi- 
mately a diameter equal to the radius of the corneal facet, 
svoo Of an inch or more. Such pencils would produce no 
definite picture even if the super-position were perfect, which 
is far from being the case when the cornea deviates from a 
37—2 
