On a Mode of reviving Dormant Impressions on the Retina. 435 



Had it been my object to give, by means of a periodic function, 

 the most exact representation possible of the observed numbers, 

 there would have been no difficulty in choosing such a function. 

 The labour at present, however, would lead to no useful result. 

 We first need a much more extensive series of observations. I 

 have therefore contented myself with an approximation sufficiently 

 close to demonstrate the existence of a period. 



It may not be amiss to remark here, that the daily motion, as 

 shown by me on a former occasion, consists of two portions 

 altogether different — of a polar wave and an equatorial wave. 

 In carrying out the subject, it will be in the first place necessary 

 to separate these portions, by comparing observations made at the 

 equator with those made in higher latitudes. That both portions 

 are subject to a contemporaneous increase or decrease does not 

 appear to me probable. 



I will remark, in conclusion, that the diurnal motion of the 

 horizontal intensity is also subject to a considerable alteration ; 

 whether of the same period, or that of the declination, I have 

 not yet ascertained. Our observations must be submitted to 

 various tedious reductions before they are in a state to decide 

 the above question. 

 Munich, Sept. 1851. 



LX. On a Mode of reviving Dormant Impressions on the Retina. 

 By W. R. Grove, M.A., F.R.S. 



To Richard Taylor, Esq. 

 My dear, Sir, May 4, 1852. 



AN experiment which I made lately on the revival of the 

 images formed on the retina by bright objects may be 

 worth recording. I have never seen it in any work which I have 

 read ; and on showing it to some gentlemen much better ac- 

 quainted with this subject than I am, they regard it as new ; if 

 not so, my note will do no more harm than taking up a very 

 small portion of your valuable space. 



1st. Look steadily at a luminous object sufficiently bright to 

 be borne by the eyes without great inconvenience, then turn the 

 eyes upon a dark body or dark space : an image of the object 

 looked at will be seen, a fact familiar to every body. When the 

 image has completely faded away and is no longer visible, pass 

 backwards and forwards between the eye and the dark body a 

 white substance, say a sheet of paper, the image will be imme- 

 diately revived, and may be thus indefinitely reproduced. 



If the light is in the first instance not sufficiently vivid to pro- 

 duce the: continued impression on the retina, but is nearly so, 

 2F2 



