9 8 4 



INSECTS AND AKACHNIDA 



a single image, resembling that which we obtain by means of our 

 single eyes. This judgment has received a confirmation as unex- 

 pected as it is complete and beautiful. The subject of the real 

 nature of compound vision can be considered no longer a matter of 

 doubt. We have as complete evidence of its character as \ve have 

 of that of vision by vertebrate eyes. It is to Professor S. Exner, 

 of Vienna, that we are indebted for the striking though simple 

 results. He has been engaged for years on cognate researches, 

 and has at length succeeded in taking a photo-micrograph of the 

 image presented at the back of a compound insect eye in precisely 

 the same manner as a similar photograph might be taken with 

 the retina removed at the back of the eye of one of the higher- 

 vertebrates. 



The demonstration was satisfactorily made, and the present Editor 

 is indebted for a knowledge of the following details to the courtesy 

 of a private communication from Professor Exner. 



The general result of the researches on the subject is presented 

 in fig. 734, which is the image at the back of the compound eye 



of Lampyris splendidula 

 (fire-fly), in the position 

 in which it would be por- 

 trayed upon the retina, but 

 magnified 120 diameters. 

 On to the window pane a 

 letter R cut out in black 

 was fixed ; the distance of 

 the window from the eye 

 was 225 cm., while the dis- 

 tance of the church from the 

 window through which it is 

 seen in the magnified image 

 was 135 paces. 



The result is unmistak- 

 able ; there may appear to be 

 some matters of interest still 

 needing interpretation, but 

 these are explained in the 

 monograph by Exner, giving 

 the complete details of the 

 method he adopted and the 

 mathematical explanation of 

 the results he obtained. The 

 rectitude of the image and 

 the reversion of the R are 

 certainly noteworthy ; and 

 as a contribution to our 

 knowledge of the physiology 



of sight in insects and other animals with compound eyes, the im- 

 portance of the result obtained by the ingenuity and skill of Professor 

 Exner is great, giving us a new start on solid ground. The mathe- 

 matics of the question are fully discussed by Exner in a memoir, to 



FIG. 784. Image of a window with the 

 letter R on one of its panes, and a church 

 beyond, taken through the compound eye 

 of Lampyris splendidula, and magnified 

 120 diams. 



