FURTHER REMARKS ON THE COMPOUND EVE. 579 
twenty-five facets, which accords with the calculated diameter 
on the view which I have taken of its origin. 
Under all the theories which have been suggested, only very 
small pencils of light can enter or traverse the great rods, and 
such pencils could only produce a very feeble reflection, or a 
very minute one. The minute image so produced can indeed 
be seen with an ophthalmoscope in the centre of a dark disc, 
which in most insects replaces the luminous reflection above 
described. The dark disc is known as a pseudo-pupil, the 
nature of which will now be discussed. 
The Central or Principal Pseudo-pupil—Exner says [252]: 
“I now come to the consideration of a remarkable optical 
phenomenon, the attempted explanation of which has cost me 
many a headache. Once more Leydig first observed the 
phenomenon, but it was scarcely discussed until Thompson 
Lowne again observed it. Lowne remarked in a butterfly 
(Pieris brassice) six black spots arranged around a central 
spot, and explains it as a diffraction image.’ 
These spots are seen with the naked eye or with a simple 
lens, and whatever the position of the insect the central spot is 
exactly opposite the observer’s eye. The central spot is the 
primary or principal pseudo-pupil, and I think it is undoubtedly 
due to the fact that the light which falls directly upon the 
cornea is feebly reflected, whilst that which falls on it 
obliquely is reflected more powerfully. In other words, the 
cornea in the region of the central pseudo-pupil is seen as a 
transparent object, and the dark pigment beneath it absorbs 
the light, whilst the surrounding cornea is seen as an opaque 
object, and by reflected light. 
If the central pseudo-pupil is observed with an ophthalmo- 
scope adapted to a microscope, from which the eye-piece 
has been removed, with a powerful light —direct sunlight 
is best—a small central bright spot is seen within the dark 
pseudo-pupil. This phenomenon is described by Exner. The 
light by which this bright spot is seen undoubtedly traverses 
the great rod, and is, I think, reflected by the retinal rods; 
by careful focussing, the image of the retinula (in sensu mihi) is 
