PRINCIPAL USE OF FACETTED EYES 259 



experiments, and claims that while the compound eye is inferior to the verte- 

 brate eye for making out the forms of objects, it is superior to the latter in dis- 

 tinguishing the smallest movements of objects in the total field of vision. 



More recently Mallock has given some optical reasons to show that Miiller's 

 view is the true one. He concludes, and thus agrees with Plateau, that insects 

 do not see well, at any rate as regards their power of denning distant objects, 

 and their behavior certainly favors this view. It might be asked, What advan- 

 tage, then, have insects with compound eyes over those with simple eyes ? 

 Mallock answers, that the advantage over simple-eyed animals lies in the fact 

 that there is hardly any practical limit to the nearness of the objects they can 

 examine. " With the composite eye, indeed, the closer the object the better the 

 sight, for the greater will be the number of lenses employed to produce the im- 

 pression ; whereas, in the simple eye the focal length of the lens limits the dis- 

 tance at which a distinct view can be obtained." He gives a table containing 

 measures of the diameters and angles between the axes of the lenses of various 

 insect eyes, and states that the best of the eyes would give a picture about as 

 good as if executed in rather coarse woodwork and viewed at a distance of a 

 foot, " and although a distant landscape could only be indifferently represented 

 on such a coarse-grained structure, it would do very well for things near enough 

 to occupy a considerable part of the field of view." 



The principal use of the facetted eye to perceive the movements of 

 animals. Plateau adopts Exner's views as to the use of the facetted 

 eye in perceiving the movements of other animals. He therefore 

 concludes that insects and other arthropods with compound eyes do 

 not distinguish the form of objects ; but with Exner he believes that 

 their vision consists mainly in the perception of moving bodies. 



Most animals seem but little impressed by the form of their enemies or of their 

 victims, though their attention is immediately excited by the slightest displace- 

 ment. Hunters, fishermen, and entomologists have made in confirmation of this 

 view numerous and demonstrative observations. 



Though the production of an image in the facetted eye of the insect seems 

 impossible, we can easily conceive, says Plateau, how it can ascertain the exist- 

 ence of a movement. Indeed, if a luminous object is placed before a compound 

 eye,'it will illuminate a whole group of simple eyes or facets ; moreover, the cen- 

 tre of this group will be clearer than the rest. Every movement of the luminous 

 body will displace the centre of clearness ; -some of the facets not illuminated 

 will first receive the light, and others will reenter into the shade ; some nervous 

 terminations will be excited anew, while those which were so formerly will cease 

 to be. Hence the facetted eyes are not complete visual organs, but mainly organs 

 of orientation. 



Plateau experimented in the following way : In a darkened room, with two 

 differently shaped but nearly equal light-openings, one square and open, the 

 other subdivided into a number of small holes, and therefore of more difficult 

 egress, he observed the choices of opening made by insects flying from the other 

 end of the room. Careful practical provisions were made to eliminate error ; 

 the light-intensity of the two openings was as far as possible equalized or else 

 noted, and no trees or other external objects were in view. The room was not 

 darkened beyond the limit at which ordinary type ceases to be readable, other- 

 wise the insects refused to fly (it is well known that during the passage of a thick 

 cloud insects usually cease to fly). These observations were made on insects 



