436 Mr. J. Cockle on Algebraic Transformation, 



the invisible image may be brought out or first rendered visible 

 by moving the white object between the eye and the dark body 

 or dark space looked at. The white substance should be in a 

 situation exposed to light so that its whiteness affects the eye, 

 and not held in shadow. After a little practice, it is astonishing 

 to what an extent and for how long a time images may be thus 

 reproduced. 



2ndly. Reverse the experiment, looking from the bright object 

 at white paper, and a dark image of the object will be seen ; 

 when this has faded away, move between the eye and the paper 

 a dark substance held so as to reflect as little light as possible 

 to the eye, and the image is reproduced on the white paper, or 

 may be in the first instance produced as with the converse expe- 

 riment. 



The explanation which occurs to me is, that the effect is one 

 of contrast between the portions of the retina which have not 

 been strongly affected and those which have. 



The white paper dulls or deadens the sensitive portion of the 

 retina for an instant, more than the part which has been previ- 

 ously rendered non-sensitive to other impressions than that 

 which it has received by the bright light, and the black super- 

 venes as a contrast to the parts affected by the white, but not 

 to those unaffected. In the converse experiment, the black 

 relieves or renders more sensitive the comparatively unaffected 

 portions of the retina, but has little or no operation on the non- 

 sensitive parts ; thus at the moment of removing the black body, 

 the unimpressed portions of the eye are affected by the white 

 substance, but the impressed portion is comparatively dead to it. 

 Probably coloured bodies looked at, and coloured screens moved 

 to and fro, would give a series of complementary effects ; and if 

 I find the point has not been examined, I may with your per- 

 mission add a few experiments to the two given in this note. 

 I remain, clear Sir, 



Yours very truly, 



W. R. Grove. 



LXI. On Algebraic Transformation, on Quadruple Algebra, and on 

 the Theory of Equations. By James Cockle, M.A., of 

 Trinity College, Cambridge ; Barrister-at-Law of the Middle 

 Temple*. 



THE following are intended as supplementary to previous 

 observations of mine in this Journal, 

 (a.) Since the relation (/) (at p. 291 of vol. ii. S. 4) gives 



x m+1 y + ax m =0, zy m+l + ay m =0, ...(/) 

 * Communicated by the Author. 



