PHYSICS. 223 



singing or speaking to the film when in proper tension, 

 beautiful figures appear, which may be reflected from the 

 film directly on a screen. 



Mann has described in Nature an improved method of pro- 

 jecting Lissajous curves upon the screen. On a base-board 

 two reed boxes are placed, one horizontal, the other vertical, 

 capable of slight adjustment, so arranged that the reeds face 

 each other. These reeds are inserted in reed plates, clamped 

 to the face of the boxes, the vertical one giving the funda- 

 mental note, the horizontal consisting of a series giving all 

 the intervals up to the twelfth. The reeds themselves are 

 similar to those used in harmoniums. To each, about an 

 inch from its free end, a small mirror of silvered glass is at- 

 tached. By means of an air current, controlled by two taps, 

 the reeds may be thrown into vibration, the rate being con- 

 trolled within certain limits by the air pressure. A beam 

 of lioht beins: thrown on one mirror is reflected to the oth- 

 er, and thence to the screen. By admitting the air blast 

 the reeds are thrown into vibration, and the figure character- 

 istic of the ratio represented by the reeds is produced. 



Terquem has proposed a new method of projecting the 

 curves of two forks vibrating rectangularly, known as Lissa- 

 jous figures, which he says renders it impossible to produce 

 these figures even by means of the calcium light. Upon one 

 of the forks, placed vertically, is fixed, at the end of one of 

 the prongs, a small square plate of aluminum, in which a mi- 

 nute hole is pierced with a fine needle. Upon a prong of the 

 other fork, which is supported horizontally, with its plane par- 

 allel to that of the first, is placed a small lens having a focal 

 length of three or four centimeters. This lens is attached to 

 a screen of aluminum, which is screwed to the one prong, a 

 counterpoise being placed on the other. On strongly illu- 

 minating the minute opening, and placing the second fork 

 so that its lens forms a sharp image of this opening on the 

 screen, extremely sharp and well-defined curves are obtain- 

 ed, whose amplitude exceeds that given by mirrors. 



Tisley has improved his compound pendulum apparatus for 

 drawing curves, and now gives to it the name harmonograph. 

 It is capable of giving a great variety of curves, since paral- 

 lel and elliptic motions can be combined in it with rectangu- 

 lar vibrations. Each pendulum is independent, and one of 



