STUDIES IN THE RETINA. 459 



then be seen obviously fading away, the shape being re- 

 tained, but tlie bright colour and sharpness of contour have 

 disappeared, and the whole appearance suggests their being 

 slowly absorbed. Although, as above stated, with the excep- 

 tionof the ellipsoid (and the oil globule in thecoues of the frog), 

 I have never found refractive matter in the inner limbs in 

 Amphibia, cases occur elsewhere in the animal kingdom in 

 which large inner limbs become filled with it, but in a manner 

 entirely confirmatory of my argument that its source is the 

 pigment epithelium. The clinging of pigment granules to 

 the protoplasmic walls of cones was noted in Part I. 



Agaiu, in a series of sections of retinas of animals which 

 had been exposed for three hours to the light of an arc lamp,^ 

 the heat rays being screened off as far as possible, one 

 interesting result is conspicuous. The pigment epithelium 

 is here and there disorganised, and isolated pigment cells 

 have forced their way up to various heights among the rods. 

 These can be found in all stages of losing their pigment; 

 some appear as nuclei still thickly enveloped in pigment, 

 others with only a trace of pigment, while here and there 

 nuclei alone persist from which all the pigment and the 

 protoplasm have disappeared. Fig. 12 shows in a tangential 

 section, selected because of the cross-sections of the rods, 

 such a nucleus, bereft of all its pigment, embedded among 

 rods, and in these latter the reticulum has been compressed 

 into the axis, which, as above suggested, indicates the absorp- 

 tion of extraneous matter through the walls. 



Other effects of this exposure to such a fierce light have 

 still to be studied. For instance, the contents of the rods 

 have a singularly blotchy appearance, but I cannot satisfy 

 myself whether this lies in the object or in the accidents of 

 staining. 



While these arguments are, I think, sufficient for the 



1 I am indebted to my friend Mr. George Newtli, of the Royal College of 

 Science, not only for the use of the necessary apparatus, but also for indis- 

 pensable advice and assistance in making a series of experiments with pure 

 monochromatic light, the results of which are still being worked out. 



