90 Rev. J. Challis on the Aberration of Light. 



cell, there is also much difference in the aggregate of the gra- 

 nules composing particular cells. For example, intermingled 

 with cells of a dark hue, there are others less deeply tinted, 

 which give the tissue in which they are found a mottled ap- 

 pearance. This fact is well-illustrated in the hair, and also 

 in the nails, in which latter it is no uncommon thing to find 

 an isolated streak produced by the accumulation of a number 

 of cells containing coloured granules, in the midst of colour- 

 less cells. 



When pigment granules are examined separately, they 

 offer very little indication of the depth of colour which is pro- 

 duced by their accumulation. I have observed some to have 

 the hue of amber, while others scarcely exceeded the most 

 delicate fawn. The depth of colour of the deep stratum of 

 the epidermis in the Negro, is evidently due to the composi- 

 tion of that layer of these granules, while the grayness of the 

 superficial layers of the same tissue results not merely from 

 the desiccation of these granules, but also from the fact of 

 those subsequently produced being less strongly coloured, and 

 also from the addition of a colourless cell-membrane. 



The epidermic scale of the Negro has a mottled appearance, 

 from the numerous secondary nuclei and their attendant co- 

 loured granules which are scattered through its texture. 



P.S. Since my communication of the above paper to the 

 Royal Society, I have confirmed its truth by further observa- 

 tions, and have ascertained that the same principle of growth 

 is applicable to the formation of mucus and pus-corpuscles. 



December 1845. 



XIX. On the Aberration of Light, in Reply to Mr. Stokes. 

 By the Rev. J. Challis, M.A., Plumian Professor of Astro- 

 nomy in the University of Cambridge*. 



r |^HE remarks Mr. Stokes has made on my Explanation of 

 -*- the Aberration of Light, since they have little reference 

 to the more important parts of the communication, require 

 from me but brief notice. 



I agree with all Mr. Stokes has said about the direction of 

 vision through a telescope, but cannot perceive what it has to 

 do with aberration. In selecting the wire of an astronomical 

 telescope for the terrestrial object to which the direction of 

 the celestial object is referred, I had not the least reference to 

 vision through a telescope. It would have answered my pur- 



* Communicated by the Author. 



