THE STROBOSCOPE EFFECT 199 



8. There is no noticeable change in pressure with time, which 

 shows the absence of air leak into the apparatus, in spite of the 

 presence of two stopcocks that might be expected to afford some 

 leak. This result may be laid to a careful regrinding with fine 

 emery and water before use. 



THE STROBOSCOPIC EFFECT BY DIRECT REFLEC- 

 TION OF LIGHT FROM VIBRATING MIRRORS. 



L. E. DODD. 



A very simple and convenient method of producing the 

 stroboscopic effect is to reflect light directly from a vibrating 

 mirror upon a stroboscopic screen. The mirror may be such 

 as is afforded by a vibrating membrane which is itself reflecting 

 or has a suitable mirror attached either directly or indirectly 

 to it. 



Any stroboscopic apparatus is divided into two principal 

 parts, the stroboscopic screen arrangement with its similar fig- 

 ures in motion, and some means of obtaining periodic glimpses 

 of the screen. The latter is commonly provided by some method 

 of periodic illumination with suitable frequency. The mano- 

 metric gas flame is the device most commonly used 'to pro- 

 duce periodic illumination, although a periodic electric spark, 

 or a discharge tube with tuning fork interruptor of the induc- 

 tion coil primary, give good results. The important condition 

 of illumination for producing the stroboscopic effect is that there 

 shall be in any given small region of the screen the periodic 

 change in light intensity. (It is not necessary that the light be 

 at any time entirely reduced to zero intensity.) Given such 

 a small region on the screen and the similar figures of suit- 

 able size, the stroboscopic effect will occur in this region if the 

 latter undergoes a periodic change in the intensity of the light 

 falling upon it, regardless of how this change is produced. 



There are in general two possible ways in which the light in- 

 tensity in the region of small area can change: (1) by a change 

 in the intensity of the beam of light as a whole, which falls on 

 the screen, with the beam itself possessing at any instant a uni- 

 form intensity over its cross-section, or (2) by a periodic back 

 and forth lateral displacement of a beam whose intensity does 



