THE STROBOSCOPIC EFFECT 201 



Professor Bell's explanation, in view of his results with both 

 the vibrating mirror and the vibrating lens, of the light effed 

 on the selenium cell may be taken as at least partial explana- 

 tion of the phenomenon. However, in both cases of mirror and 

 lens, as the cross section of the beam alternately contracts and 

 expands, there must be a lateral displacement increasing in 

 amount toward the margin of the beam. If the beam is not of 

 perfectly uniform intensity over its cross section, and it is not 

 likely to be, at any particular fixed point where the beam falls 

 there will be changing intensity due not only to its expansion 

 and contraction, but also to the fact that a little element of 

 the beam incident at this fixed point is being replaced by an 

 adjacent element of different intensity, different because of 

 the non-uniformity of the beam. Because of the relatively small 

 amplitude of the membrane the lateral displacement of the beam 

 cannot be very large, especially at short distances, and yet it is 

 sufficiently large to give the stroboscopic effect even in the case 

 of spots of 3 or 4 mm. diameter on the stroboscopic screen, and 

 that at relatively short distances. It cannot be supposed there- 

 fore that, the lateral displacement is sufficient to cause an ele- 

 ment of the beam to sweep clear across one of these larger spots. 



It would seem that we cannot be at all certain without further 

 investigation as to just how important a role this effect due to 

 a non-uniform beam plays in the experimental results of Bell 

 with a selenium cell. We may, however, be certain that it is 

 present in both the case of the vibrating mirror and that of the 

 vibrating lens. 



One is justified in suspecting that it does play some part in 

 Bell's results, in view of the results in stroboscopy obtained by 

 the author. For the stroboscopic effect has been found to exist 

 very sharply when the vibrating mirror was made by silvering 

 one of the small circular microscope cover glasses and attaching 

 it to a vibrating membrane by means of a bit of cork between 

 them. Under these circumstances it is not to be expected that 

 the mirror will change its curvature when the membrane vibrates. 

 but will move as a whole. Moreover the stroboscopic effect was 

 produced with a piece of ordinary mirror izlass. attached to the 

 membrane. This mirror glass was much too thick to admit the 

 possibility of changing curvature. It appears therefore that the 

 stroboscopic effect is to be explained, in large part a1 least, by 

 the periodic lateral displacement of a beam of non-uniform in- 



