132 



SCIENTIFIC RECREATIONS. 



turning round its centre. And what is the explanation of these curious 

 effects ? Mr. Thompson does not believe (and we share his opinion) that 

 the faculty possessed by the retina of preserving images during a certain 

 time (persistence of impressions on tJie retina} can entirely explain these 

 phenomena. Without desiring to formulate a decided theory, Mr. Thompson 

 is of opinion that we may class these facts with others which have been 

 known for some time, and that perhaps it is necessary to attribute to the 

 eye some new faculty which may explain the whole .at once. 



Brewster and Adams have described phenomena which are equally 

 curious, the principal of which we will describe, adding also some analogous 

 investigations due to Mr. Thompson. The result seems to be that there 

 exists in the eye a badly-defined purpose of nature, which in a certain way 

 compensates (Brewster) for the real phenomenon, because it has a contrary 



Fig. 137. Another figure of Mr. Thompson's. The different circles appear to turn round 

 if we give the design a rotating movement. 



effect, which will continue for some time after the cessation of the pheno- 

 mena, and which gives by itself a sensation contrary to that which the real 

 movement would have produced. Thus, after having fixed our eyes for- two 

 or three minutes on a rushing waterfall, if we suddenly turn our glance on 

 the adjacent rocks, the latter appear to move from top to bottom. It is not 

 a question' here of the effect of the relative movement to be observed on regard- 

 ing simultaneously the falling water and the rocks ; if one can succeed in 

 abstracting oneself to such an extent that the water appears motionless, the 

 rocks appear to take a contrary movement. In the phenomenon we describe 

 there is no simultaneous comparison ; the eyes are turned successively first 

 on the water, and then on the rocks. In a rapid river, such as the Rhine 

 above the fall at Schaffhausen, the stream is not equally swift in every part, 

 and the current is noticeably more rapid in the middle of the river than 

 earn the banks. If we look fixedly at the centre of the stream, and then 



