Photography of Hippies. 413 



Description of the Apparatus. 



A rectangular wooden trough, about 1 centim. deep and 

 12 x 15 centim. area, contains the mercury upon the surface 

 of which the ripples are produced. This trough rests upon 

 the rectangular base of a retort-stand, the upper portion of 

 whose standard is bent at right angles over the base so as to 

 be parallel with the latter. This horizontal portion is passed 

 through a rectangular block of wood, and the whole is then 

 slung from a gallows-like wooden structure one metre high. 



The spark-gap from which the light proceeds to illuminate 

 the mercury surface is placed near the top and to one side of 

 the wooden stand. Light from this gap falls upon a lens to 

 the left of the stand, so that the emergent light is parallel ; 

 after reflexion the light is collected by a second lens similar 

 in all respects to the first, so that an image of the spark would 

 be produced at the primary focus of the second lens ; the 

 focal length of these lenses was 44 centim. The camera is 

 placed in such a position as to enable the first achromatic lens 

 of the combination to collect the rays and converge them so 

 as to come to a focus at the aperture in the lens stop. The 

 stop used was the smallest of the set belonging to the camera 

 (F G4). The camera is then focussed upon a line thread laid 

 on the surface of the mercury. 



In addition to the first spark-gap, which was about *5 

 centim. across, a second gap was used in order to increase the 

 brightness of the spark in the first gap. The second gap was 

 varied from time to time, but was generally 1*5 centim. across. 

 The first gap was shunted by a piece of stout thread soaked 

 in calcium-chloride solution. This prevented small sparks due 

 to induction. The knobs of a Wimshurst machine were con- 

 nected, one with a terminal of the second spark-gap and the 

 inside coats of a battery of four half-gallon leyden-jars ; the 

 other Wimshurst terminal was connected to the outer coats, 

 one side of the first spark-gap, and to earth. A wire joining 

 the other terminals of the two gaps completes the spark 

 arrangements. 



Method of Causing the Ripples. 



The ripples are due to the agitation of the surface of the 

 mercury by a style of glass attached to one prong of a vibra- 

 ting tuning-fork. The fork was in most cases struck with a 

 rubber-shod hammer ; but in the last two experiments, in 

 which a strip of cover-glass attached to the fork acts as a line- 

 source, it was found that the irregular large waves caused by 

 the concussion entirely masked the phenomena which it was 

 sought to photograph. In these cases the fork was main- 

 tained in synchronous vibration with another similar fork 



Phil. Mag. S. 5. Vol. 43. No. 265. June 1897. 2 I 



