OP SENSATION AND THE SENSES. 489 



at a distance or close does not depend upon the structure of the eyes ; 

 every compound eye which distinctly discerns objects at a distance, 

 produces also closely a clear image of it. But the larger the individual 

 facets are, and the smaller the spheres formed by them, and the 

 brighter the pigment deposited between the lenses, the more indistinct 

 does the image of the object seen become, and in such a structure a 

 better image is formed of distant objects, but a worse one is seen of 

 approximate objects, for the rays are more diverging in consequence of 

 their proximity, whereas they run more parallely from every point of an 

 object at a distance ; in the former case, therefore, it passes through 

 the brighter pigment into the contiguous glass lenses, and renders 

 obscure the image that should be there formed upon the retina. The 

 apparent size of the object seen corresponds only with its true size 

 when the convexity of the eye is perfectly spherical and concentrical 

 with the convexity of the optic nerve ; in every other case the apparent 

 size of the image will not correspond with its true size, and the image 

 must therefore appear distorted. Hence all elliptical or conically arched 

 eyes w il see worse than those forming the segment of a circle. As 

 the structure of the eye does not differ in water insects and those 

 which avoid the light from that of day insects and those which live 

 upon the land, namely, the pigment is by no means brighter in the 

 former, as Marcel de Serres affirms, consequently their sight must fully 

 correspond with the sight of day insects. 



With respect to the difference of structure of the eyes in larva? to 

 those of the perfect insect, in insects with an imperfect metamorphosis, 

 it consists especially in the relative size of their compound eyes. These 

 are always smaller in larvae, but continue increasing with every moult, 

 until they at last attain their full size. In the large eyes of the larvae 

 of the Cicada no facets are observed ; these, therefore, gradually dis- 

 tinctly develope themselves. The cornea of the eye is changed also 

 with the change of skin, which very well admits of a transformation. 

 Whereas simple eyes are never found in larvae with an imperfect 

 metamorphosis ; they present themselves only as bright spots where 

 they are subsequently to appear. The majority of larva? of insects 

 with a perfect metamorphosis have merely simple eyes, and, indeed, 

 exactly where the compound eyes afterwards appear; many entirely 

 want eyes, and a few, as the larvae of the gnats, have already compound 

 eyes. With respect, therefore, to the development of the eyes during 



