1 26 Discoveries of Miiller and others i?i the 



structure, that they do not possess any visual perception of 

 circumscribed forms. These lower animals see only the light, 

 which will be more or less clear, obscure, or coloured, accord- 

 ing to the intensity and kind of light predominating. If this, 

 in the true sense of the word, simple eye, assigned to the An- 

 nehdes, apparently consisting only of a transparent layer or 

 cornea, covering a portion of nervous matter, or retina, is to 

 have the power of distinguishing the relations of locality, it will 

 be necessary that the light coming from any one point of an 

 external object shall not be transmitted to all parts of the 

 retina situated in the same plane. For example ; if the nervous 

 or sentient organ bespread out over a flat surface, then variously 

 coloured rays of light, coming from the border of a semi- 

 circle, will, it is true, illuminate simultaneously all the parts 

 of such a retina ; but the different kinds of rays falling on the 

 retina at different angles of incidence, there will be developed, 

 in addition, certain colours in different parts of the retina. If 

 the sentient surface or retina be spherical, the specific action 

 of the light will be most intense at the point where the light 

 falls on the surface in the direction of the ray, and one side of 

 the sphere will in no degree participate in the colours of the 

 other. This would be the highest point to which this grade 

 of organisation of the eye could attain. 



There are two modes of organisation possible and necessary, 

 for the sentient surface to become a perfect visual organ, in 

 which the differences of colour in external objects shall be re- 

 produced upon the retina. 



1. By refraction, by means of refracting media or lenses. 



Rays from external objects fall upon the whole of the 

 exposed exterior of the eye. Each portion of the surface of. 

 the eye is in contact with the different kinds of rays which 

 approach it in every possible direction ; but, in traversing the 

 transparent parts of the eye, the divergent rays from each 

 point of emission are again converged into distinct points or 

 foci at a certain distance, where the sentient organ or retina 

 is situated. These points of reunion, these foci of identical 

 rays, are found therefore upon the retina, in an order corre- 

 sponding precisely with the points of emission on the exterior 

 object ; and this reunion of the rays in foci at a determinate 

 distance, by means of refraction, produces distinctness of 

 image on the retina. Such is the case in the eyes of all the 

 vertebrated animals, in the MoUusca, in the Cephalopodes, 

 and some Gasteropodes, in the ^rachnides, &c., and in the 

 stemmata of insects. 



When the retinae of two eyes of this kind are organised in 

 such a manner that their different parts are in precisely the 



