of the Permanence of lmpressio7is on the Retina. 451 



be necessary, as it is easy to convince oneself, to substitute for 

 the black disc pierced with three slits another black disc 

 pierced with five slits. 



Designating again by V',; and V'„ the velocities of the two 

 discs relative to the partial case which wehave just examined, 

 we shall have evidently, for the general relation between two 

 equivalent systems, one of which would belong to this same 

 case, and the other to the case in which, the velocities being 

 always in the same direction, that of the transparent disc would 

 be less than that of the black disc. 



^-1 = 1- 



' n 



which gives 





^'+^=2 (9.) 



In the system to which the term ^ of this expression is re- 



ferred, V,^ must, we know, exceed V^ only by one unit; we 

 may then in the place of V^ substitute V„— 1, and the above 

 expression may then be stated under the form 



v ~ V ' 



» M 're 



whence it is seen that, in the supposed relation r^j- reduced 



* n 



to its simplest expression, V'„ will be equal to V„, and V^ will 

 be a unit greater. 



There now remains the third partial case, namely Vrf=2V„. 

 This is of an entirely peculiar nature, and seems to me very 

 remarkable. The absolute value of M is then, in fact, equal 

 to unity ; so that the angular dimensions of the distorted figure 

 are equal to those of the regular figure, or, in other words, 

 the deformation consists only in a reversing of this last figure. 

 Reciprocally, then, if upon the transparent disc is drawn 

 any regular figure, the instrument will reproduce this same 

 figure, but reversed, that is to say, having on the left what 

 on the disc is to the right, and vice versa. If, for example, 

 upon the transparent disc there be drawn a head viewed in 

 profile and looking to the right, the instrument will reproduce 

 identically this same head, but turned so as to look to the left. 

 In short, if the drawing traced upon the transparent disc be 

 such that its right half is symmetrical to its left half, if, for 

 example, this drawing represents a head seen in front and 

 illumined in front, or a word formed of letters placed symme- 

 trically, and which do not change by reversing, such as the 



