276 



OPTICAL PROJECTION 



Fia 145 



N of the lantern. A wire pointer c is easily affixed to the 

 centre of one side of the mirror by sticking it in a piece of 

 cork E, which has a groove cut in one side, by which it is 



attached to the mir- 



<v> N ror. Letting the 



beam from a hori- 

 zontal slit in the 

 optical stage strike 

 the centre, it will be 

 readily seen that if 

 it impinges at right 

 angles it is reflected 

 back ; but that at any 

 other angle, the angle 



(measured from the normal or perpendicular, as indicated 

 by the pointer) of the reflected ray is equal to that of the 

 incident ray. A whiff of smoke near the mirror will show 

 the beam brightly if required. 



154. Use of Parallel Beams. This experiment being an 

 example of a large class, in which we want a distinct pencil or 

 beam of light, it is well to consider the various methods of 

 manipulation in order to produce it. The figure shows the 

 simplest, which will answer for many experiments, but not for 

 all. A horizontal slit is here placed in the stage of the 

 optical front. This slit may be of black card, or in zinc or 

 brass, or in a disc of thin metal fixed by a spring wire in 

 one of the wooden frames used for polariscope slides, the 

 standard size for all slides used in the optical front being 

 4x2^ inches. Or an ' adjustable slit ' made to fit the stage 

 may be used. For this experiment a broad slit should be 

 used, not less than 3 mm. wide. A beam so produced is not 

 parallel, but sufficiently so for many purposes. 



With the electric light, or a good mixed jet, it is better to 

 remove the objective, place a slit or aperture on the front of 

 the open nozzle, and use the ' parallel beam ' through this. 



