106 THE COCKROACH. 



the brain combines these impressions into some kind of picture, 

 a picture like that which could be produced by stippling. It 

 may be added that the movements of the insect's head or body 

 would render the distance and form of every object in view 

 much readier of appreciation. No accommodation for distance 

 would be necessary, and the absence of all means of accommoda- 

 tion ceases to be perplexing. Such is Miiller's theory of what he 

 termed " mosaic vision." Many important researches, some 

 contradictory, some confirmatory of Miiller's doctrine,* have 

 since been placed on record, with the general result that some 

 modification of Miiller's theory tends to prevail. The most 

 important of the new facts and considerations which demand 

 attention are these : 



Reasons have been given for supposing that images are formed 

 by the cornea and crystalline cones together. This was first 

 pointed out by Gottsche (1852), who used the compound eyes of 

 Flies for demonstration. Grenacher has since ascertained that 

 the crystalline cones of Flies are so fluid that they can hardly 

 be removed, and he believes that Gottsche's images were formed 

 by the corneal facets alone. He finds, however, that the 

 experiment may be successfully performed with eyes not liable 

 to this objection, e.g., the eyes of nocturnal Lepidoptera. A bit 

 of a Moth's eye is cut out, treated with nitric acid to remove the 

 pigment, and placed on a glass slip in the field of the micro- 

 scope. The crystalline cones, still attached to the cornea, are 

 turned towards the observer, and one is selected whose axis 

 coincides with that of the microscope. No image is visible 

 when the tip of the cone is in focus, but as the cornea 

 approaches the focus, a bristle, moved about between the mirror 

 and the stage, becomes visible. This experiment is far from 

 decisive. No image is formed where sensory elements are 

 present to receive and transmit it. Moreover, the image is 

 that of an object very near to the cornea, whereas all observa- 

 tions of living Insects show that the compound eye is used for 

 far sight, and the simple eye for near sight. Lastly, the treat- 

 ment with acid, though unavoidable, may conceivably affect the 



* A critical history of the whole discussion is to be found in Grenacher's "Seh- 

 organ der Arthropoden" (1879), from which we take many historical and structural 

 details. 



