388 



SCIENCE. 



[N. S. Vol. IX. No. 220. 



ever a cam axle is to be removed. This is 

 done for the front axle, for instance, as 

 follows : Let the metal strap at c be 

 loosened and the pin therein withdrawn ; 

 this frees the end C. Now let the spring 

 latch at D be withdrawn and the axle of 

 pulley J^ slid to the right. This frees the 

 end D. The cam axle may now be with- 

 drawn to be replaced by another on re- 

 versing these operations. 



3. Cam axles. — Each of the cam axles 

 (Figs. 1 and 2) carries 25 eccentrics of thick 

 tin plate, equidistant, about 1" apart and 

 dififering in phase by y^ circumference in 

 Fig. 1, so that in this case there are two 

 complete right-handed turns in each of the 

 helices. The diameter of the rear eccen- 

 trics is 4", with a double swing of 3" ; the 

 diameter of the front eccentrics is 3", with 

 a double swing of 2", but this series has 

 an advantage of position or leverage, as 

 will presently be seen. A safe minimal 

 margin of h" beyond the axle is thus left in 

 each case. 



It is usually convenient to keep the rear 

 axle in place. In the room of the front axle, 

 however, the other right-handed helices 

 (Fig. 2), containing respectively 1 or 3 turns 

 to the whole length ; another containing one 

 right-hand and one left-hand helix (the 

 eccentrics alternating), and a final one left- 

 handed, with 4" cams and 3" throw, corre- 

 sponding to the rear axle (see Fig. 6), are 

 provided. The two latter are adopted for 

 the illustration of rotary polarization. The 

 three former are a means of obtaining wave- 

 length ratios 1:1,1:2,2:3 for all ampli- 

 tudes, periods and phases on removing the 

 front axle only. 



The general purposes of the machine will 

 not require more axles than this, though I 

 have used others to be referred to below. 



The eccentrics themselves of the heavy 

 tin plate specified are turned together to a 

 common size on the lathe, and soldered to 

 the axle by aid of a suitable gauge. This 



need merely be a piece of board of a width 

 corresponding to the distance apart of the 

 cams, and having the phase angle carefully 

 marked on both sides. If the board is per- 

 forated normally for the reception of the 

 axle, and cut across axially so as to be re- 

 movable, the soldering of the cam axles is 

 surprisingly easy. I have also tried other 

 methods with success. The work must be 

 done expeditiously, as prolonged heat warps 

 the cams. 



The helices shown in the figure are usually 

 right-handed screws. Since they are sta- 

 tionary, a wave advancing from the operator 

 corresponds to counter-clockwise rotation. 

 This is an apparent disadvantage as com- 

 pared with left-handed stationary screws, 

 but as the waves in the former case advance 

 from left to right (positively for the observer 

 in front) for clockwise rotation by an oper- 

 ator on the right of the machine the dispo- 

 sition chosen is preferable. 



4. Levers, Riders and Balls. — To obtain the 

 different types of wave motion from the 

 cams described, long extensible levers of 

 thin brass tube are provided, shown in de- 

 tail in Fig. 8 (longitudinal dimensions ^, 

 cross dimensions h), and in place in the re- 

 maining figures. 



The levers wereoriginally made of heavy 

 guttered tin plate behind and light guttered 

 tin plate in front. Latterly, however, I re- 

 placed these by the light extensible ' curtain 

 rods ' of verj' thin brass tube,* consisting 

 of a round tube E snugly telescoping into a 

 wider round tube FF, about 5/16" in diam- 

 eter. The first tube E is provided with an 

 axial pin R, 3" long, carrying a 1/2", cork 

 ball Q (painted red), representing one of 

 the vibrating particles of the wave. The 

 rod R is not seen in sunshine shadows and 

 is added for this reason. Its end is tipped 



*■ These ' rods ' are in the market, each about two 

 feet long, thus admitting of a safe extension to much 

 over three feet. Though made of thin split tube, 

 they fit well. The price is trifling. 



