THE THEORY OF ARTHROPOD VISION. 569 
was poised, how does he know the insect could not detect the 
change? How does he know that the insect was not curious 
to know what the finger was which had suddenly obtruded 
itself on its consciousness ? 
When I accepted Miiller’s theory, and believed that each 
ommateum produces only a single visual stimulus, I calculated 
the possible sharpness of vision for the central part of the 
visual field of a Blow-fly as Jy of that of man; this means that 
objects seen as distinct by man at sixty feet distance would 
be so to the insect one foot from the eye. On the view I now 
hold, as each retinula consists of seven rods, three in a line, 
the acuity of vision would be 35. So that objects twenty feet 
from the eye of man, which are recognised as distinct, would 
be so distinguished at one foot by the insect. Objects at 
half an inch would correspond to those seen by man ten inches 
from the eye, whilst objects y'5 of an inch from the eye of the 
insect would appear as they do to man under the microscope 
with an inch objective. By the same calculations, the sharp- 
ness of vision in the Dragon-flies, the Bee, and the Wasp is 
from eight to ten times greater, or about 3° S., or half that in 
man. Such sharpness in vision might well account for the 
manner in which these insects find their nests or their prey. 
Exner by a different method, by the direct investigation of 
the images, arrived at the conclusion that, in the Glow-worm, 
Lampyris splendidula, the acuity of vision is about gly; but as 
his methods undoubtedly diminish the clearness of the image, 
the sharpness of vision is probably greater. 
In the above attempt to compare the sharpness of vision of 
an insect with that of man, the assumption has been made that 
each retinal rod forms a single visual area, and that the dis- 
tinctness of perception is determined by the number of retinal 
rods which the image covers. This is the only possible hypo- 
thesis which we can assume with our present knowledge; but 
there is evidence that one retinal rod may give rise to a com- 
plex sensation. In small birds the retinal rods (cones) are as 
large, or even larger, than they are in man, so that as the images 
formed upon them must be smaller in direct proportion to the 
