Visual Organs of Insects and Crustacea, 1^6 



which the light must undergo in the stemmata or simple eyes 

 of the articulated animals must be very considerable; for, 

 even if the spherical lens be nearly of the same density as 

 the cornea, and if, in consequence, its anterior surface should 

 scarcely increase the refraction already produced by the con- 

 vexity of the cornea, still it is very certain that the posterior 

 surface of the lens will, on account of the medium of inferior 

 density with which it is in contact, cause the light to undergo 

 a strong refraction there. The refraction will, of course, be 

 much greater than in fishes, on account of the power of the 

 refracting media in these latter being much diminished by their 

 being surrounded by a fluid of greater density than the air. 



In the simple eyes of the articulated animals the rays of 

 light undergo a fourfold refraction : first, through the convexity 

 of the cornea ; secondly, through the still greater convexity 

 and increased density of the lens ; thirdly, through the pos- 

 terior convexity of the lens in the hollow space or canal com- 

 prised between the vitreous and crystalline bodies ; and, 

 fourthly, through the anterior and convex surface of the vi- 

 treous body. This organisation indicates the absence of a 

 clear vision of distant objects ; but, from the same circum- 

 stance, the existence of a clear and distinct vision of near 

 objects ought to be inferred. 



If we consider that the larvae of insects, whose sphere of 

 existence may, from their mode of organisation, be said to be 

 limited to the objects immediately around them, have only 

 stemmata, and that they have no vision of objects placed at 

 distances to which it is not necessary for them to move, it 

 becomes highly probable that the simple eyes of spiders, &c., 

 are also myopic. These organs are to the compound eyes, as 

 it regards vision, what the palpi are to the antennae, as it 

 regards the sense of touch. The larvae have no antennae, but 

 generally they have palpi ; for the most part they possess no 

 compound eyes, but commonly they are provided with stem- 

 mata. When the simple eyes are of different magnitudes, the 

 smaller ones seem to be specially destined for the vision of the 

 nearest objects, and particularly for the organs of manducation. 



The visual horizon of the simple eyes is certainly very 

 small ; the light which falls laterally or obliquely upon these 

 organs will, it is true, undergo a powerful refraction; but, 

 beyond a certain point, even these lateral rays will be inter- 

 cepted by the zone of choroid pigment placed in front of the 

 vitreous body. It is the smallness of the visual field of each 

 of the simple eyes individually, and the position of these 

 organs, always more or less divergent from each other, that 

 alone prevent the several fields of vision from comprising 



K 3 



