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 M tiller'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 W V 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 ^V 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 -^ 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 f 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 ^V ; 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 



