578 THE SENSES AND SENSORY ORGANS. 



Exner ascribes the appearance to a reflection from a tapetum 

 behind and between the great rods ; he further considers that 

 the disappearance of the reflection is due to an alteration in 

 the disposition of the iris pigment. 



There are, I think, many objections to the view that the 

 reflection originates from the great rods or from the tracheal 

 tapetum, if this term is to be applied to the tracheal network 

 between and behind the great rods, especially as this tapetum 

 is least developed in those very insects in which the reflection 

 from the eye is most brilliant, and is most developed in those 

 insects, as the majority of Diptera, in which there is no 

 vestige of luminous reflection. The insects in which the 

 luminous reflection is most brilliant are, however, all dis- 

 tinguished by the large size of their crystalline cones. Now, it 

 is manifest that the reflection of the light which enters the eye 

 will be most marked where the refraction of the rays is most 

 powerful, and where the highly-refractive medium is concave 

 towards the source of light. The convex surface of the cornea 

 disperses the light which is reflected from it ; but the apex of 

 the crystalline cones is virtually a concave mirror, and the light 

 reflected from it through the corneal lens will leave the eye 

 as a pencil of parallel, convergent, or slightly divergent 

 rays. 



Such a reflection will only occur from the internal surface 

 of the crystalline cones where these are not covered externally 

 by dark or black pigment. In the nocturnal Lepidoptera 

 which have been kept in the dark the iris pigment is with- 

 drawn from the apex of the cone, so that a considerable por- 

 tion of its surface acts as a concave reflector. I find that 

 the reflection from this surface corresponds with a luminous 

 image of the cone apex, subtending an angle of 20 to 25 

 from the optic centre of the corneal lens. As the adjacent 

 lenses have an angular divergence of about one degree, the 

 images produced will overlap and intensify each other. 



Hence the light which emerges from the insect's eye gives an 

 image of a bright disc, apparently situated behind the cornea. 

 The diameter of this disc is equal to that of from twenty to 



