442 COMPOUND EYES OF ARTICULATA. 



correspond in function with the single eye of the Vertebrate 

 animal. For no rays except those which correspond in direc- 

 tion with the axis of each cone, can reach the fibre of the 

 optic nerve at its apex ; all others being stopped by the layer 

 of black pigment which surrounds it. Hence it is evident 

 that each separate eye must have an extremely limited range 

 of vision, being adapted to receive but a very small collection 

 of rays proceeding from a single point in any object ; and as 

 these eyes are usually immoveable, animals with but a small 

 number of them would be very insufficiently informed of the 

 position of external things. But by the vast multiplication in 

 the number of the eyes, and the direction of their axes to 

 every point in the hemisphere, their defects are compensated j 

 a separate eye being provided, as it were, for every point to 

 be viewed. And it is quite certain, from observation of the 

 movements of Insects, that their vision must be very perfect 

 and acute. 1 



575. Although these Compound Eyes exist in all Insects 

 and in most Crustaceans, Spiders and Centipedes, they are in 

 general not the only organs of vision which these animals 

 possess. Most of them are also furnished with several simple 

 eyes, analogous in their structure to those of higher animals, 

 but les? complex and perfect in their organization ; these, 

 which are for the most part disposed on the back of the head, 

 are largest in Spiders. The larvas of some Insects possess the 

 simple eyes without the compound; the latter being only 

 developed at the time of the last metamorphosis. The simple 

 eyes of Insects do not appear to be nearly so efficient as 

 instruments of vision, as are their compound ones ; for when 

 the latter are covered, the animals seem almost as perplexed 

 as if they were perfectly blinded. Simple eyes, closely re- 

 sembling those of Insects in structure, are found in most of 



1 It is commonly believed that each of these compound eyes pro- 

 duces its own image of the same external object, as do our two eyes; 

 but from the description here given of their separate directions when 

 united, it is evident that in no two of them can an image of the same 

 object be formed at the same time. The membrane formed of all the 

 lens-like cornese united together, when separated from the other parts 

 of the eye, and flattened-out, has the properties of a multiplying-glass, 

 each lens forming a distinct image of the same object ; but this is not 

 the case when they are arranged in their natural position, because no 

 two of them have the same direction. 



