Prof. Magnus's Hydraulic Researches. 171 



If only one spoked wheel be employed, in conjunction with a 

 disc placed behind it and provided with circular holes, when the 

 disc and the wheel revolve in opposite directions, the holes no 

 longer appear round, but oblong ; and by using different discs, it 

 is easy to convince oneself that the outlines always appear of a 

 different form to what they are in reality. 



150. The anorthoscope described by M. Plateau, as well as 

 the other very ingenious contrivances which this physicist has 

 given for similar metamorphoses, depend, as is well known, upon 

 this alteration in the outline. 



151. In order to make such alterations in outline visible, I 

 have had an apparatus constructed by means of which two discs 

 of the most different kind may easily be made to revolve in the 

 same or opposite directions with the most variable velocities. 

 The discs are made of thin pasteboard ; so that any number of 

 them, very various in respect to the shape and number of their 

 orifices, may be easily procured. If such discs be covered with 

 a solution of shell-lac, they do not absorb moistui-e from the air, 

 and are very durable. Each disc has its own axis ; but both lie 

 in the same horizontal line. Each axis receives its motion by 

 means of a cord passing over a roller. The two rollers are 

 parallel, but are placed vertically against a large brass plate, 

 against which they are pressed by springs. On turning the 

 brass plate, the rollers which are pressed against it turn also 

 through friction. One of the two rollers may be moved along 

 its axis. If the system is so arranged that the two rollers touch 

 the brass plate on opposite sides of its centre, they revolve in 

 opposite directions. If they both lie on the same side, they turn 

 in the same direction. The nearer the moveable roller is to the 

 middle of the plate, the smaller is its velocity. This principle 

 of motion has already been employed by Mr. Faraday*, in a 

 somewhat different form, for a similar purpose t- 



* PoggendorfF's Annalen, vol. xxii. p. 601. 



t The ai)i)aratus which I have had made is shown in PI. I. fig, 8. a h and 

 c d are the two axes with their discs A B and C D. The axis « Z> is set in 

 motion by means of the strap /^ : the latter passes over the roller /;, which 

 is furnished with a projecting rim. Its axis is fastened in the frame z z, by 

 which it is pressed by the spring k against the brass plate M N. The axis 

 erf is set in motion in the same manner by means of the strap h k passing 

 over the roller q. The axis w n of the latter fits into the frame x x. The 

 roller itself has no projecting rim ; instead of this, there is upon the same 

 axis m n the small metalhc disc r, which is somewhat greater than the 

 roller q. The disc r may be moved along its axis mn, so that it can be 

 brought between m, the centre of the plate M N and n, or between m and 

 q. The frame xx in fastened at // between points, and is so pressed by 

 the springs / 1 against the plate M N, that when the latter moves on its axis 

 «, the <lisc r is moved by friction, and sets its axis m «, as well as the roller 

 q, in motion. 



In order to give the plate M N the necessary velocity, a piuiou is faat« 



