ANATOMY AND LIFE HISTORY OF MOLLUSCA. 151 



tapetum, the image of any object in front of the pupil will be seen 

 with the greatest distinctness, diminishing in definition according 

 as the objective of the microscope is raised or lowered. But the 

 rods also lie above the tapetum, so that upon them the image must 

 be formed. A remarkable phenomenon may be observed, by 

 focussing between the argentea and the place where the image 

 formed by the lens is seen with the greatest distinctness, for there 

 one sees a double image, less distinct towards the argentea, but 

 increasing in sharpness towards the focal point of the eye, where 

 the two images ultimately fuse to form a single one. The only 

 explanation I have to offer for the origin of this second image is 

 that it is a reflected one of the first, formed by the curved surface 

 of the argentea. A plain mirror would never reflect an image 

 formed by a lens, since the rays of light would be dispersed ; 

 neither would the image be reproduced by a concave mirror, unless 

 the curvature was such that the divergent rays coming from the 

 lens, impinged upon the reflecting surface, at right angles to the 

 tangent at that point. In that case each reflected ray would 

 coincide with the incident one, and a reflected repetition of the 

 lenticular image would be reproduced, both being formed at the 

 same point. The exact relation between the focal distance of a 

 lens like that of Pecten, and the radius of the concave mirror 

 which would again unite the rays, has not been determined. (PI. 

 32, -fig. 149)." 



It will serve to make the references more complete if I here give 

 a very interesting section of another kind of eye in which the various 

 organs are singularly clear and interesting. It is a simple eye of an 

 embryo of a Cephalopod. It was a section made by Dr. Haswell 

 of the Sydney University, who very kindly communicated it to me 

 that I might subject it to microscopic examination and make the 

 accompanying drawing. The section is one of an embryo Octopus 

 in the egg and probably just before extrusion ; at any rate the 

 stage is that at which the organs have become fully formed and 

 the yolk-sac has been entirely absorbed. The eggs were obtained 

 in Port Jackson, but unfortunately the species from which they 

 were taken was not determined and of course no specific character 

 can be made out from the embryo. At pi. x., fig. 15, I have 

 given a drawing of one of the sections, or rather that portion of 

 it which includes the eye-chamber as far as it has been developed. 

 The explanation of the letters in the plate will make the section 

 easily intelligible, a cornea or integument, b iris, c ciliary body, 

 d aqueous chamber, e internal rods of the retina, / pigment-layer, 

 g external layer of rods, h body of optic ganglion, i i are what are 

 called the white bodies, which Dr. Ray Lankester believes to be 

 the material at the expense of which the optic ganglion is nourished; 

 j lens, but for further information on this somewhat technical 



