172 Mr. H. M. Bernard on 



the brilliance of the light. Every shadow, every shade of 

 a shadow, every dark moving object, every black line of the 

 book we are reading represents so many reliefs, so many 

 degrees, so many shiftings, and so many durations of relief 

 from the pressure which the granules are exerting laterally 

 upon the sensory nerve-fibrils in some way incorporated with 

 the retinal rods. 



Colour- Sensation. — No theory of colour-sensation can be 

 satisfactory unless it can be shown to be a natural develop- 

 ment — that is, a development, without sudden break or sudden 

 addition of new factors, of ordinary light-sensation. It is 

 satisfactory, therefore, to find that colour- sensation almost 

 naturally follows from the foregoing description of general 

 light-sensation. 



That there exists some connexion between the granules 

 and the formation of the cuticular rods is not only 

 probable itself, but can even, I believe, be demonstrated 

 under the microscope. We need not now discuss the 

 details : it is enough for our purpose if this cuticular structure, 

 the rod, varies slightly in texture in such a way as to be 

 almost glassy near the external limiting membrane, and from 

 this point to consist of zones in which corpuscles of increasing 

 size (though always microscopic) are suspended. That the 

 rod has some definite texture tending to cause it to break 

 transversely into short lengths, histologists are agreed. We 

 are further justified in assuming some heterogeneity in order 

 to avoid the total internal reflection of the light down to its tip, 

 which would take place if the rod were a homogeneous glassy 

 structure. In addition to this specialization into zones, with 

 different-sized corpuscles suspended in the substance of the 

 rods, we have only to assume that, of the sensory nerve- 

 fibrils embodied in e; ch rod, one or more terminate among the 

 finest corpuscles, one or more among the next coarser, and so on 

 to the tip, where the coarsest are found. Colour-sensation 

 would, it seems to me, naturally result from such an arrangement. 

 We require no more movement among the pigment-granules 

 than we required for the appreciation of the ordinary variations 

 in light and shade. The red rays, according to the law 

 illustrated daily in the sky, passing through all zones con- 

 taining the smaller corpuscles, would be caught and dispersed 

 on all sides by the largest granules at the tips of the rods. 

 The pressure of the granules already crowded in the " gang- 

 ways " would be immediately directed both from above and 

 below to the point where the red light is breaking in from 

 the side. Again, rays of shorter wave-lengths would be 

 caught by the smaller suspended corpuscles and scattered 



