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of ants. It is at once noticed that the ants, which seem to 

 dislike coloured light, not only get out of the range of the visible 

 spectrum, but also avoid the space be3-ond the violet filled with 

 the ultra-violet rays. These latter e.xerting, as is well known, 

 a powerful chemical influence, it may be assumed, however, that 

 it is the chemical rather than the optical action of the ultra- 

 violet light which brings about this curious effect. 



In my further experiments I started from the supposition 

 that the ultra-violet rays would be perceived also in composite 

 sunlight ; or, an eye which is at all sensitive to them would 

 not remain impassive if subjected to sunlight deficient in those 

 rays, but would probablv notice the difference between com- 

 plete sunlight and a light from which all the ultra-violet rays 

 had been absorbed by means of a proper medium. Glass 

 has this (quality, and one would think that an eye sensitive to 

 those rays would surely notice the difference between an open 

 and a closed glass window. I have, however, observed that 

 Diptera, as well as Hymenoptera and Lepidoptera, fail to find 

 in a window divided into a great number of panes those that 

 are open, unless they are guided by a draught of air. It appears 

 therefore evident that these insects have no clear perception of 

 the difference between glass and air. All these results, which 

 go far to show that the difference between the single and com- 

 pound eye is not one of principle but merely one of degree (as 

 is also the case with the eyes of different species of vertebrates), 

 seem to be opposed by the anatomical structure of the composite 

 eye, which one might suppose would render the physiological 

 process of seeing quite different. With regard to this objection 

 I may be allowed to remark that the \'ery act of seeing, involving 

 the composition, substitution, and transmission of the image, 

 remains as yet absolutely unexplored and unexplained, even 

 with regard to the eyes of vertebrates and man. 



The interpretation of the physical image by the mind remains 

 an unsoKcd riddle, and the ophthalmologist is daily confronted 

 with phenomena which he so far fails to explain. Normally 

 we see only a single image, although we possess two eves. In 

 a certain disease of the eye we see three images; in another 

 kind of disease the part of the retina reflecting the image of 

 whatever appears in the field of vision is here and there replaced 



