406 ME. B. THOMPSON LOWNE ON THE COMPOUND VISION 



outer portion of the organ, which does not collapse like the rest of the spindle. This 

 view is supported by a comparison of the outer end of the spindle in the Fly (fig. 19) 

 with the structure in question. 



C. The Great Bods. 



The great rods consist essentially of the spindles and their cellular sheaths. The more 

 important modifications of these structures have already been described, in their relation 

 to the alterations which they undergo after death, and I have nothing to add further 

 with regard to the very remarkable modifications which have been described by myself 

 and others, except that they result from post-mortem changes. 



The most important point in relation to the theory of Arthropod vision is, however, 

 the direction of the axes of the great rods. It is well known that these are often seen 

 to be strongly curved, even in the most carefully prepared sections, and this fact has 

 been brought into prominence by the opponents of Midler's hypothesis * . Such a curved 

 condition of the rods and spindles would be still more fatal to my own view. I have 

 therefore carefully investigated this point, and conclude that this curvature is the result 

 of changes of tension in the parts of the dioptron, and of the elasticity of the spindles. 

 Sections made through the entire eye, immediately after the death of an insect, show no 

 such curvature of the axes of the great rods when examined as opaque objects with low 

 powers ; yet the eyes of the same species exhibit very strongly curved rods when sections 

 of the eye are examined in balsam. The manner in which the isolated rods twist and 

 curl in all fluids shows that they must be powerfully affected by the action of the fluids 

 usually required for the preparation of sections, and their action could scarcely fail to 

 produce contortion of the great rods, even in the closely packed condition in which they 

 exist in the cavity of the dioptron. The curvature would chiefly affect those rods which 

 are nearest the periphery of the eye — a condition seen in all the sections which I have 

 examined. The slightest pressure on the cornea in the recent eye permanently distorts 

 the great rods. Hence it can hardly be expected that the axes of these structures would 

 be undisturbed, at least around the periphery of the eye, in specimens preserved in any 

 fluid which affects the normal tension of the parts. 



I have made numerous attempts to determine the optical relations of the dioptric 

 structures of the compound eye by means of a modification of the ophthalmoscope, but 

 at present I have not been able to throw any further light upon the functions of the 

 great rods by this means ; except that the colour of the reflex obtained appears to 

 depend on the colour of the fluid contents of the spindle. 



I have found the best method of examining the reflex to be the substitution of a 

 reflecting ophthalmoscope for the eyepiece of a microscope. By this means a bright 

 luminous spot may be observed as a real image in the tube of the instrument. A 

 quarter objective must be used, and the mirror of the ophthalmoscope must be strongly 

 illuminated. The microscope is then focussed so that a real image of the corneal facets 

 is seen between the objective and the eye of the observer. By bringing the object-glass 

 gradually nearer to the insect's eye the reflex will come into view. 



* Exner. Biolog. Centralblatt, i. p. 273. 



