the Permanence of Imp'essions on the Retina. 533 



viz. that to which the attention should be directed^ may be the 

 only one exhibited ; to effect which^ it will suffice to intercept, 

 by means of a screen, the light of the lamp, except over the 

 space occupied by the erect image. This screen should be placed 

 behind the transparent chsc as near to it as possible, and parallel 

 to its plane ; lastly, the lamp should be so aiTanged, that the 

 flame is opposite to, and at a distance of 6 to 7 centimetres 

 from, the apertui-e intended to give passage to the light ; this 

 aperture ought to have the form of a trapezoid terminated late- 

 rally by two straight lines directed towards the axis of the disc, 

 and above and below by horizontal lines ; it will easily be found 

 what angular width and what height must be given to this 

 aperture ; the screen may be made of blackened pasteboard. 



But the employment of this screen supposes that the erect 

 image remains always precisely in the same place, which requires 

 that the relation of the two velocities should be rigorously exact 

 and perfectly invariable. Now it is almost impossible to realize 

 this condition by a system of pulleys and strings*. A system 

 of toothed wheels must therefore be substituted, and the following 

 is the arrangement which I have adopted in my instrument. 



The horizontal axes, on which are respectively fixed, by means 

 of nuts, the transparent and the black disc, have a certain interval 

 between their extremities which face each other ; each of these 

 opposite extremities has a wheel furnished with teeth perpendi- 

 cular to its plane, and turned toward the space which separates 

 the two wheels ; these two wheels arc therefore vertical, parallel, 

 and moveable around the same perpendicular. The one whose 

 axis is furnished with the transparent disc, has a diameter of six 

 centimetres ; the diameter and the number of teeth of the one 

 whose axis receives the black disc are four times less. In the 

 interval left between the two wheels passes a steel rod, capable 

 of turning upon itself, and furnished with a pinion, the upper 

 part of which catches in the upper part of the large wheel, and 

 its lower part in the upper part of the small wheel. It is clear, 

 that when the steel rod is turned, the two wheels, and conse- 

 quently the two discs, turn in contrary directions, and tliat the 

 velocity of the Ijlaek disc is four times that of the transparent 

 disc. The two discs are eighteen millimetres distant from one 

 another. The steel rod descends to the foot of the instrument ; 

 towards its lower extremity it has a second pinion, which gears 

 into another vertical wheel ; and the axis of this last is furnished 

 with a small winch, by means of which the system is set in 

 motion. 



The glass chimney of the lamp should be surrounded with a 

 plate-iron chimney of twice the diameter, in which, at the height 

 * Sec the preceding Note. 



