THE COMPOUND EYE OF INSECTS AND CRUSTACEA. 
27 
And further, notwithstanding the differences of size, form, 
and position of the bacilli of the insect eye as compared with 
those of the vertebrate eye, an important agreement in respect 
to the physical character and molecular arrangement of their 
substance has been recently noticed by Schultze. The sub- 
stance of the vertebrate retinal rod has been discovered to be 
composed of matter having different refractive power, which is 
so disposed that the length of the rod is made up of discs piled 
on each other. Under favourable circumstances, a faint stria- 
tion of cross lines may be seen when the retinal rod is examined 
under the microscope and is in perfectly fresh condition. In 
the club-shaped swelling, and indeed throughout the length of 
the nerve fibre of the insect compound eye, a similar striation 
is readily seen (the insect rod being larger and thicker) ; but on 
no other nerve structure, as far as is yet known, does this pecu- 
liarity exist. The cross markings are solely due to the different 
refractive power of the alternate elements or discs by the super- 
position of which the rod is formed. The fact is especially 
important, as it promises a further clue to the explanation of 
the physical conditions of vision. Hitherto the optical image 
has always been considered the final point to which physical 
investigation could conduct us. But in the newly discovered 
properties and composition of the retinal rod (which has been of 
late years universally acknowledged to be the percipient ele- 
ment), a step further towards the physical analysis of visual 
phenomena seems to be gained ; namely, the mode in which 
physical impressions of light may be transferred to the perci- 
pient element. Between the physical impression of light and 
the actual sensation or sense of sight there will always probably 
remain a chasm which no physiological enquiry can ever bridge 
over. But it is possible to conceive a differential process in the 
transmission of light impressions which may serve to render 
our ideas of what is perceived and how perceived more in- 
telligible. And perhaps the perception of colour, to an explana- 
tion of which these newly discovered facts apparently point, 
may receive a better elucidation. If such hopes be fulfilled, it 
may fairly be inferred from the close analogy of the bacillar 
structure of the insect compound eye with the retinal rods of 
the vertebrate eye, that a similar perception of colour as well 
as form may be predicated of the insect as well as of man 
himself. 
We should be paying an ill compliment to the readers of the 
Popular Science Keview by offering any apology for the lengthy 
and minute details of the structure and function of the insect 
eye which we have here attempted. The day is passed when 
popular science was supposed to be acceptable only when it was 
superficial and unscientific ! 
