580 THE SENSES AND SENSORY ORGANS. 
seen in optical section. This image is rose-coloured or green. 
I have seen such an image in all the insects’ eyes I have 
examined in this way; it is brilliant in the Diptera, which do 
not exhibit the phenomenon of pseudo-pupils. 
The central pseudo-pupil is small, intensely black, and circular. 
In this it differs entirely from the pseudo-pupils of the second 
and third order, which are larger, dim, and irregular in form. 
All the pseudo-pupils appear to be beneath the cornea. 
Pseudo-pupils of the Second Order.—These are six in number ; 
they are twice as large as the central pseudo-pupil, and gray in 
colour. I have little doubt these are diffraction images; they 
correspond with the angles of a hexagon circumscribed around 
the central pseudo-pupil as its centre. 
Pseudo-pupils of the Third Order—These are approximately 
twice the diameter of those of the second order, more irregular, 
and grayer. They are twelve in number, but generally only 
two or three are visible at once, as they fall outside the com- 
pound cornea. The whole circle can never be seen at the 
same time, but they may be brought into view in succession by 
causing the central pseudo-pupil to fall on the edge of the 
cornea, and to travel round it by moving the insect. 
The pseudo-pupils of the second and third order correspond 
with the diffraction images produced by a hexagonal grating. 
Exner gives a totally different explanation of the pseudo- 
pupils of the second and third order, and ascribes them to the 
manner in which the pigment of the chamber is arranged. 
For his explanation I must refer the reader to his mono- 
graph [262]. 
It will be seen that whatever the true explanation of the 
pseudo-pupils may be, the phenomenon throws no light on the 
manner in which images are formed on the retina, although the 
bright image of the retinula seen with the ophthalmoscope 
and microscope combined tends towards the verification of the 
hypothesis I have adopted in these pages, since it shows that a 
real image of the retinula can be produced in front of the eye, 
just as a real image of the vertebrate retina can be seen with 
the ophthalmoscope. 
