> yofthe Permanence of Impressions on the Retina. 449 



Jshat, for the result produced in a revolution of one of the slits 

 to be superposed on those of the preceding revolutions, the 



relation ■^— must also be an entire number. Let us take, for 



' rt 

 example, the velocity of the transparent disc equal to six times 

 that of the black disc. The absolute value of M will then be 

 5, and consequently the angular dimensions of the distorted 

 figure will be, as in the published anorthoscope, five times 

 those of the regular figure, Tliis distorted figure then will 

 be constructed in the same manner, the inversion excepted, as 

 in the system of the published anorthoscope ; whence it fol- 

 lows, that if, for example, we by transmission look at one of 

 the distorted figures which belong to this system, we shall 

 have that which will produce the same regular figure in the 

 new system, on turning the disc, that is to say, by placing 

 opposite the eye the face of this disc which was first 

 turned toward the light. Moreover, in this new system, the 

 regular figure will be likewise repeated five times around 

 the centre ; for when, after a coincidence between one slit and 

 the first point of the distorted figure, this slit shall have per- 

 formed one-fifth of a revolution, the point in question will 

 have performed six-fifths of a revolution, that is to say, one 

 revolution plus one-fifth, so that it will be found behind the 

 slit, and consequently a regular figure will have been produced. 

 As to the number of the slits to pierce in the black disc, they 

 must be six instead of four ; for when, after a coincidence 

 between one slit and the above point, this point shall have 

 accomplished one revolution, the slit will only have reached a 

 sixth of its revolution. 



According to what we have stated above, it is seen that if 

 we desire, with the same distorted figures, to obtain absolutely 

 identical results in the two systems, it will suffice, in order to 

 employ them in the second, to invert the discs which had been 

 constructed for the first; that is to say, that each of these 

 discs shall be attached to the pulley, so that the face which, 

 in the first system, was turned towards the black disc, should 

 on the contrary be turned towards the lamp ; moreover, it 

 will be necessary in one of the systems to employ a black disc 

 pierced with four slits, and in the other a black disc pierced 

 with six slits. U, in passing from one of these systems to the 

 other, the inversion of the transparent discs is not effected, it is 

 clear that it is then the regular figures which will return ; if, for 

 example, we employ successively in the two systems, and with- 

 out inverting it, the distorted figure intended to give, as a 

 regular result, horses at a gallop, the horse which shows itself 

 on the upper part of the image will have, in one of the systems, 



