1883.] Structure and Functions of the Eyes of Artliropoda. 143 



It is well known that if an object-glass is placed in the reversed 

 position beneath the stage of a microsoope, and the instrument 

 is then focussed for its posterior focal plane, it can be nsed as 

 a telescope. I regard the Dioptron as an aggregation of similar 

 optical arrangements ; the Corneal lens corresponds to the inverted 

 objective, and the Spindle to the microscope. 



I have made and given a series of measurements of the parts of an 

 insect's eye in support of this view ; the focal lengths of the corneal 

 lenses, those of the spindles, and their relative distances, from each 

 other, as well as the number and size of the corneal images are con- 

 sistent with this theory. Therefore a continuous picture, a mosaic of 

 erect magnified central portions of the several subcorneal images, falls upon 

 the retina, and the sharpness of vision is not necessarily dependent on 

 the number of corneal facets. 



The complex modifications of the Dioptron appeared at first in 

 many cases to offer insuperable objections to this view, and this 

 necessitated a very careful reinvestigation of these structures, more 

 especially in relation to the changes which they undergo in the pre- 

 paration of sections for microscopic observation. 



These researches have shown that many of the modifications 

 observed are due to differences in the nature of the material of which 

 the refractive elements are composed, not only in different genera 

 and families, but even in the same species in different stages of 

 development. 



In many cases the refractive media consist of an oil-like fluid, 

 which is decomposed or dissolved in the process of preparing the 

 object for microscopic examination ; in other cases the media 

 consist in part at least of practically indestructible Chitin. And 

 further the great elasticity of the parts gives rise to profound modifi- 

 cations the result of alterations of tension. 



In the first, or introductory portion of my paper, I have reviewed 

 the work of my predecessors with the object of showing the relation 

 of previous observations to the theory which I have enunciated. The 

 remainder of my communication is divided into four parts. 



I. The Structure and Functions of the Dioptron. 



II. The Structure and Functions of the Neuron. 



III. The Development of the Compound Eye. 



IV. The Morphology of the Eyes of Arthropods. 



I. The Structure and Functions of the Dioptron. 



In this portion of my paper the structure of the Dioptron is 

 described, and its relation to the views I have adopted is discussed. 



Perhaps the most important additions to our knowledge of this 

 organ has been the discovery of the very important part played by 

 the oil-like fluid already alluded to. This fluid is easily decomposed, 



