146 On a Wave-apparatus for Lecture-Purposes. 



To describe the action of the apparatus, it should be pre- 

 mised that the starting-point of each crank is when the crank 

 and rod are in the same straight line. Thus, regarding for 

 the moment the first left-hand crank only, it will be at the 

 starting-point, or zero, when the crank 6 has moved on 45° to 

 the left. Similarly with the first right-hand crank, it will be 

 at zero if moved back through 45° to the right ; so that, in the 

 position shown in the figure, the left-hand crank has moved 

 through seven eighths of a revolution, while the right-hand 

 crank has moved through one eighth ; or the difference of 

 their phases is six eighths, or three quarters ; and on turning 

 the cranks, a wave corresponding to circularly-polarized light 

 is produced, each bead describing a circle in succession. Bring 

 now the cranks into the position of the figure, unscrew the 

 pin p f , and, keeping the left-hand crank steady, move the 

 right-hand crank to the vertical position upward. Now the 

 left-hand crank, as before, has made seven eighths of a revo- 

 lution, but the right-hand crank five eighths, i. e. a difference 

 of phase of one quarter. On inserting the pin p f and now 

 turning the cranks, the beads will describe the same wave as 

 before. Now keep the left-hand crank in its first position (the 

 position in the figure), remove the pin p f , and turn the right- 

 hand crank until it is at 45° from the vertical below and to 

 the left. The left-hand crank, as before, has made seven 

 eighths of a revolution, the right-hand one two eighths, or 

 there is a difference of phase of five eighths ; and on now 

 inserting the pin and turning the cranks, the wave produced 

 corresponds to an elliptically-polarized ray. Of course, if the 

 difference of phase be made one eighth or any odd number of 

 eighths, the same kind of wave is formed. Again remove the 

 pin, keep the left-hand crank in its first position, and turn the 

 right-hand crank to 45° above the horizontal and to the right. 

 Insert the pin; and on turning the cranks a plane vertical wave 

 will be produced, the cranks being in similar positions, each 

 starting at seven eighths of its revolution. Finally, adjust the 

 right-hand crank till the difference of phase is half a revolu- 

 tion, when a plane horizontal wave will be produced on turn- 

 ing the cranks. 



I take the opportunity of acknowledging the assistance of 

 my pupil, Mr. William Hall, who made several parts of the 

 apparatus in a simple and ingenious manner. 



