OBJECTIONS TO OTHER THEORIES. 167 



On the other hand, Boll,* Exnei-,t and Grenacher 

 seem to me to have proved that the compound eyes of 

 insects cannot act as ours do ; that the theory which 

 assumes that each facet acts as a separate eye and 

 projects an image on a retina, is physically untenable. 



In the first place, there are cases — for instance, 

 Forflcula, Dytiscus, and Stratiomys among insects ; 

 Ligia and many others among Crustacea — where the 

 cornfse are nut sufficiently arched to give any distinct 

 image. But even where an image is thrown by the 

 cornea, it would be destroyed by the crystalline cone. 



In certain Crustacea the crystalline cones are 

 elongated and curved ; this, which Oscar Schmidt j 

 regarded as fatal to Miiller's theory, is, on the con- 

 trary, as Exner has pointed out, quite compatible with 

 it, but, on the contrary, cannot be reconciled with the 

 theory of an image. 



There are few beetles in which the cornea give 

 better images than in the firefly (Lampyris spUndidula). 

 On the other hand, the crystalline cones entirely 

 destroy these images. If the eye is looked at through 

 a microscope, and the crystalline cones are left in situ, 

 the field of view appears perfet'tly black, with a bright 

 spot of light at the end of each cone. No trace of an 

 image can be any longer perceived. In fact, the 

 images seen by Leeuwenhoek and Gottsche are thrown 

 by the cornea only. 



In most cases, then, it would appear that the image 

 formed by the cornea is destroyed by the crystalline 



» "Beit, zur Phys. Optik," Arch. fur Anat. Phys. und Wiss. Mediein., 

 1871. 



t " Ueber das Sehen von Bewegungen und der Theorie des 

 zusammengesetzten Auges," Sitz. K. Akad. d. Wiss. Wien., 1875. 



X Ibid., 1876. 



