ARTHROPODAL VISION. 6l 



the same side have exactly the same axis no two can receive their rays 

 from the same point of an object, and thus, as each compound eye is 

 immovably fixed upon the head, the combined action of the entire aggre- 

 gate will probably afford but a single image, resembling that which we 

 obtain by means of our single eyes. This judgment has received a con- 

 firmation as unexpected as it is complete and beautiful," etc., etc. 



Then follows a description of Exner's experiment, with a 

 reproduction of the photooraph and some diagrams. 



It seems incredible that Dr. Dallinger could have made 

 such a statement after reading Professor Exner's book, 

 which is filled with discussions of the extraordinary refrac- 

 tions of the cones or pyramids, with the express purpose of 

 showing that other than axial rays can and do pass through 

 the cones and enter into the resultant images. 



Dallinger's image seems to agree better with Exner's 

 apposition image or Mtiller's mosaic. 



The image on the vertebrate retina may also be regarded 

 as a kind of mosaic. Here ever>^ point of the image is 

 formed by a cone of rays received upon the cornea from a 

 corresponding point of the object viewed, and the assemblage 

 of points of different degrees of illumination and different 

 colors makes the picture. 



It may seem that the arthropod with compound eyes has 

 advantages not possessed by the vertebrate. The division of 

 its eye into many units ensures practically parallel bundles of 

 rays from near objects, and hence no accommodation is 

 needed. If any of the images are distinct, they are all dis- 

 tinct, or, if a part of the image is distinct, the whole is dis- 

 tinct, without regard to direction ; while man, at least, must 

 constantly change the direction of his eyes in order that the 

 rays from the most important object in his field of view shall 

 fall upon the fovea centralis, the only spot in the whole eye 

 where a distinct impression can be made ; but there is much 

 evidence that the arthropod is either half blind as to form and 

 character or hopelessly stupid. 



In connection with the question of response to light stim- 



