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OPTICAL PROJECTION 



that ingenious physicist, the optical method of M. Lissajous 

 for observing the vibrations of a fork has been preferred 

 where possible. The simplest arrangement is shown in 

 fig. 134, where a small aperture is placed in the stage of the 

 optical front, and the rays from the condensers being made 

 parallel, the aperture is focussed on the screen, giving a very 

 nearly parallel pencil of light ending in a sharp spot. This 

 proceeds from the nozzle N, and is reflected from a small 

 mirror A attached to a prong of the fork, the other prong being 

 similarly loaded. The reflected ray is re-reflected back to the 

 screen by the plane -mirror B, when the spot should be re- 



focussed to allow for 

 the extra distance 

 from A to B, which 

 blurs the original 

 image (this re-fo- 

 cussing, or else an 

 allowance for extra 

 distance caused by 

 reflections, should 

 always be attended 

 to in this class of 

 experiments). On 

 now exciting the 



fork, the spot is drawn out into a line of light ; and if 

 then the plane-mirror B be slightly turned in its socket 

 (which is better than holding it in the hand) the line is 

 drawn out into ripples CD, showing the single vibrations 

 separately. 



Very fair ripples may be projected in this way. But they 

 will be much more brilliant if the pencil attachment (fig, 95) 

 be employed, whereby much more light is sent through a 

 small aperture on the front. This aperture is then focussed 

 by a loose lens. But the best use of this focussing lens 

 demands some consideration, according to the circumstances, 



FIG. 134. Lissajons' Method 



