the Photography of Ripples. 201 



about 8 centim. across j it was found that when no elaborate 

 precautions were taken to prevent loss by brushing, the 

 spark would just not be able to leap this gap. The machine was 

 allowed to run until the electrical circuit began to give off 

 brush-discharoes ; the current driving the motor was then 

 cut off and the cap removed from the lens ; when all was 

 judged to be at rest the second spark-gap was closed. The 

 lens cap was an ebonite arm pivoted to the camera front ; it 

 could be manipulated in the dark. 



On the Use of the Lens-Stop. 



The aperture of the stop was chosen to give the best 

 results in each case. If the surface of the mercury was 

 disturbed only slightly by the waves to be photographed, a 

 small stop was used ; while if the waves had a larger amplitude, 

 the aperture of the stop could with safety be increased. In 

 some cases the stop was as great as F. 16. The effect of 

 allowing more of the cone of light transmitted to the lens to 

 pass through the diaphragm is to smooth down the surface ; 

 while using a small stop tends to magnify the apparent 

 amplitude of the disturbance. 



Photographic Considerations. 



Considered photographically the second form of apparatus 

 suffers from a defect from which the first is free. The spark 

 is on the same side of the large lens as the camera is, and 

 thus light may be reflected from the lens and affect the 

 sensitive plate. The white marks seen on some of the prints 

 are due to this cause. 



It is obvious that the effective lens of the camera must be 

 regarded as made up of the camera lens in combination with 

 the large condenser. The use of the camera lens is to make 

 the picture a convenient size on the sensitive plate. Quarter 

 plates have been used throughout. The front combination of 

 the camera lens might be removed ; the stop and spark-gap 

 would then have to be at the same distance from the con- 

 denser. The latter is, of course, alone capable of producing 

 the photographs. Thus, for full-size pictures, if F is the focal 

 length of the condenser, the distances of the plate, spark and 

 stop, and lens from the mercury surface would be respectively 

 4F, 3F,and2F. 



The plates used were Ilford Special Rapid, and were 

 developed in darkness with hydroquinone. This developer 

 has many advantages for such work. It does not stain the 

 negatives, the plates may be left in it a long time without 

 fogging, and, what is more, it tends to increase the contrasts. 



