82 VISION. 



the head of a dragon-fly you will see that even to 

 the naked eye they exhibit a grated appearance, such 

 as might be presented by a very fine sieve ; and un- 

 der a magnifier they show a surface like that of a 

 honey comb, consisting of numbers of hexagonal 

 compartments. Every compartment is the end of a 

 tube, containing a transparent substance like glass, 

 its walls lined with black, and its deep extremity 

 occupied with a sensory structure continuous with 

 the nerve of sight The effect of this arrangement 

 is obvious ; only one ray of light can pass down each 

 tube ; all lateral rays being absorbed by the black 

 walls. The sensitive structure at the bottom of each 

 tube is thus exposed to only one spot in the land- 

 scape, the spot directly opposite it; and there is 

 painted in the bottom of the eye a miniature repeti- 

 tion of the landscape which might be compared with 

 a pattern in Berlin wool, each tube containing a single 

 stitch. Here then you have undoubtedly an instance 

 of true vision a number of separate spots of light 

 appreciated at the same moment, and every spot re- 

 ferred, not to its position in the eye, but to the direc- 

 tion from which the ray of light has come. This 

 sort of eye is an optical instrument which requires no 

 focusing. There are just as many points in the 

 landscape exhibited to the animal as there are tubes 

 in the eyes, and if the landscape is distant those 



