THE SIMPLE AND COMPOUND EYES OF INSECTS. 
595 
facet, we have to do with convergent rays and not with the focal point. This points 
to some mode by which the stimulation of the nerve ending is brought about, other 
than the union of homocentric pencils in a point beneath the compound cornea. 
In Hydrophilus piceus, however, according to Exner,”' the focal point for parallel rays 
lies within the crystal cone. 
Whether each facet in the compound eye corresponds to one or to four distinct 
luminous impressions must at present, at least, remain a matter of doubt. I think, 
however, there can be no doubt that several distinct luminous impressions are trans- 
mitted from each facet in the miero-rhabdic eye of Tipula and Vespa; and there 
can be no doubt that a number of distinct luminous impressions are received by 
the ocellus or simple eye. I cannot, however, believe that the ocelli of insects can 
produce anything worthy of the name of an image, in the Diptera and Hymenoptera 
at least. The few retinal elements, their near approach to the lens, and the strong 
curves of the surfaces of the latter, are but ill adapted for more than the perception of 
light and the direction in which it is most intense. 
In the compound eye the curvature of the cornea and the number of facets agree 
well with Muller’s theory. It is true that Claparede has expressed the opposite 
opinion, but I think I shall be able to show that this is based on an incorrect 
assumption. 
Claparede has stated that if Muller’s theory were true, a hive Bee should be 
unable to perceive objects of less than eight or nine inches in diameter at a distance of 
20 feet as distinct ; but he comes to this conclusion by assuming that the acuity of 
vision is the same over the entire field. This is far from being the case in any insect 
which I have examined, with the single exception of Tipula, where it is approxi- 
mately so, perhaps. In all the other insects which I have examined, the axes of vision 
for adjacent facets make a very small angle with each other in the central portion of 
the visual field, and a much larger one at its circumference. And although I have not 
had the opportunity of examining the cornea of the hive Bee critically, in the humble 
Bee, the Wasp, Tabanus, and the great Dragon-flies, the angles made by the axes 
of adjacent facets are not more than from eight to fifteen minutes : a condition which 
would enable objects of from half an inch to an inch in diameter to be seen as distinct 
at a distance of twenty feet — an acuity of vision quite sufficient to account for all 
the observed phenomena of vision in insects. 
The method which I have adopted in calculating the acuity of vision is as follows : — 
A magnified image of the entire cornea is thrown upon a sheet of paper by means 
of a camera lucida attached to the microscope. By using low powers and appropriate 
illumination, the error from distortion of the image can be reduced to a minimum. 
The profile of the various meridians was then sketched. By drawing tangents to the 
curve, the radii of curvature of different parts of the curve are readily found. The 
ratio of the diameter of the facets to these radii gives the sine of the angle subtended 
Wien Sitzmigsberi elite, 1876, 
