THE SIMPLE AND COMPOUND EYES OP INSECTS. 
593 
of the Brachycerous Diptera, and the Dragon-flies. I shall speak of this as the 
hydro-conic eye. 
In the other form the cone is highly modified, and appears as a very complex 
tetrasome and tetraphore. I shall speak of this as the tetraphoric eye. 
I shall conclude this portion of the subject by indicating very briefly the probable 
distribution of the three forms of compound eye, as well as that of the highly complex 
semi-compound eye, which for brevity may be called micro-rhabdic. 
I have at present found the micro-rhabdic eye in the Nematocerous Diptera, the 
Hymen optera, and Hemiptera, although I have only examined the eyes of a few 
members of the Order, and these far from exhaustively. I believe that it will also be 
found in the eyes of many Coleoptera, such for instance as Bryaxis. 
The conic eye is the usual form of the compound eye in the Crustacea : at least it is 
found in the Lobster, Palsemon, and Hyperia. As already stated, it is found in the 
Nocturnal Lepidoptera, in the Sphingidse, and probably in all the Pentamerous 
Coleoptera at least. 
Judging from Leydig’s descriptions, it is also found in the Cursorial Orthoptera. 
I have at present found the tetraphoric eye only in Vanessa, Colt as, Pie vis, Gonep- 
teryx , and Acridium ; but I have not examined any other species of the Orthoptera 
and Diurnal Lepidoptera. The hydroconic eye occurs in all the Brachycerous Diptera 
and in the Dragon-flies. 
XII. On the Theory of Mosaic Vision. 
The structure of the compound eye appears to favour the view long ago expounded 
by Joh. Muller. This view is supported by the absence of lenticular facets in many 
species of Arthropods ; by the relative sharpness of vision, not only in different species, 
but in different parts of the field of the same eye, as well as by the behaviour of a 
beam of light in passing through a highly refractive rod immersed in a less highly 
refractive medium, or surrounded by black pigment. 
It is, further, the only theory which has been hitherto advanced that is competent 
to explain the phenomena when we bear in mind the relation of the recipient 
structures of the compound eye to the nerve elements beneath them. 
On the passage of a ray of light through a highly refractive rod of small 
dimensions. — In order to arrive at some knowledge of the manner in which light 
passes through the highly refractive rods of the eye in the Arthropoda, I made the 
following experiments. 
I took a capillary glass tube about y-^-ytli °f an inch in diameter, and placed it upright 
m a small transparent trough under the microscope, and filled both the vessel and the 
tube with water. The tube was an inch in length and was examined with an inch 
objective. I found that no light passed through the lumen of the tube, but that the 
section of the w r all of the tube was brilliantly illuminated. I next placed a few fine 
4 c; 2 
