396 Intelligence and Miscellaneous Articles. 



A small light mirror is fastened with wax to the vibrating body 

 in the usual way, and a telescope is sighted at a distance of about 

 two metres. Between the telescope and the mirror is fixed near 

 the latter a thin plane glass, silvered by means of Botrger's solu- 

 tion, making an angle of 45° with the vertical. This must reflect 

 more light than it transmits. 



A watch is placed on a table below the mirror so that its image 

 reflected from the silvered surface is distinctly visible in the field 

 of view of the telescope. The watch must be brightly lighted by 

 a lamp, and another lamp so placed near the telescope that its rays 

 passing through the transparent coating are reflected by the small 

 mirror into the telescope. On looking through the telescope after 

 the body has been made to oscillate, we see the second-hand strongly 

 magnified, while at each oscillation the small vibrating mirror 

 gives a short flash in the field of view. The image of the second- 

 hand moves in the opposite direction to the hand itself. By 

 following the motion of the hand it is easy to accurately determine 

 the time of the flash to a quarter of a second. If the oscillations 

 are rapid, it is of course more convenient only to observe the time 

 of every fifth or tenth oscillation. After a few observations we 

 know when the following is to be expected, and by fixing the eye 

 on the division of the dial where the second-hand will be at the 

 moment of the flash, it is possible to determine the time to within 

 a quarter of a second. We soon get accustomed to the backward 

 motion of the index, and the reading is as easy as the ordinary one. 



Instead of the semitransparent silver mirror an ordinary opaque 

 plane mirror may be used (metal of course is best), if it is so 

 arranged that the mirror on the oscillating body is visible just 

 above or below its edge. — Wiedemann's Annalen, No. 9, 1895. 



ON THE INCONSTANCY OF THE POTENTIAL REQUIRED FOR A 

 SPARK. BY G. JAUMANN. 



The author summarizes as follows the results of his investi- 

 gation : — 



I. Sparks alter the path through which they pass ; strong ones 

 in an unfavourable sense, and weak oues often in a favourable 

 sense. The path spontaneously recovers from this after-action in 

 the course of some hours. 



II. By avoiding the smallest variations of force very high 

 potentials may be obtained. 



III. When the conditions for a discharge are not quite satis- 

 fied, the discharge takes place after a delay of a few seconds to 

 several minutes. During this retardation there is a preliminary 

 process of discharge which itself is no real discharge. 



IV. During this retardation the exciting influence of variations 

 of force can be demonstrated in an incontestable manner. 



V. A moderate variation of force does not produce the spark at 

 once, but only after 01-5 seconds. It only accelerates the pre- 

 liminary process and shortens therefore the retardation. — Wiede- 

 mann's Annalen, No. 8, 1895. 



