362 OPTICAL PROJECTION 



itself need to be very carefully tested. The projection of such 

 a doubly-refracting lens (only too easy to find) illustrates well 

 the necessity for such tests in delicate instruments. 



The alternate compression and dilatation caused in glass 

 by sonorous vibration, can be readily demonstrated in the 

 polariscope, after the method of Biot. A strip of plate glass 

 about J inch thick, two inches wide, and four to six feet 

 long, is required ; and the sharp edges of this should be 

 rounded or smoothed by a file, or emery-cloth, moistened with 

 turpentine. The strip should be screwed up exactly at the 

 middle, between the two cork-lined jaws of a wooden vice on 

 the tops of a short pillar, so that the strip is horizontal, with 

 its faces in a perpendicular plane. The portion of the polari- 

 scope containing the stage must be drawn forward from the 

 polariser, so as to leave a clear space of about an inch, and 

 through this space, so as not to touch either the front or 

 back part of the instrument, the strip must be adjusted to 

 cross the field, as near the point held in the vice as possible, 

 this being the node of the bar. The polariser and analyser 

 are crossed, for the dark field, at 45, from their usual position. 

 The other end of the bar is now to be swept between the 

 thumb and fingers holding a wet flannel or other woollen 

 cloth, so that a shrill musical note sounds from the glass. 

 At once light flashes on the screen, and if a chilled glass, or 

 selenite, be placed in the stage and focussed, the colour will 

 change at every note. If the polariser is kept in its usual 

 position the glass strip must cross at an angle of 45 ; but 

 the arrangement described is more convenient. 



210. Composition of Vibrations. The method in which two 

 vibrations in rectangular planes may be compounded into one 

 resultant vibration, can be illustrated on the screen in two 

 ways. One is to project Lissajous' unison figure, by any of 

 the apparatus described in Chapter XVII., showing in suc- 

 cession the plane orbit (converted to another plane orbit at 

 right angles when one of the reeds or forks is altered half a 



