78 Scientific Intelligence. 



He points out that the discharge through the gases was an inter- 

 mittent one and the gases were only a portion of the time in that 

 condition when, according to Kirchhofi's laws, an absorption was 

 to be expected. For the remaining portion of the time the light 

 of the arc lamp employed could go through the gas completely 

 unabsorbed. Furthermore since the spectral lines in a Geissler 

 tube represent homogeneous light, an absorption could only be 

 expected, according to Kirchhofi's laws, in a very narrow spectral 

 region. In order to show it, a spectrum of a large dispersion must 

 be employed and a narrow slit, in order that no light of the neigh- 

 boring portion of the arc spectrum can intrude upon the homo- 

 geneous portion under examination. These precautions were not 

 taken by Cantor. — Ann. der Physi/c, No. 5, 1800, p. 199-200. 



J. T. 



13. On some Properties of Light- struck Photographic Plates / 

 by Francis E. Niphek. (Abstract by the author.) — A paper 

 with the above title recently published* by the Academy of 

 Science of St. Louis, deals among other things with some features 

 which seem to be new in photography. 



In a general way it is probably true that any plate, upon which 

 a camera impression has been made, may be developed either as 

 a positive, or as a negative. Suppose the object to be a white 

 design on a black ground. Let the camera be replaced by a 

 printing frame, containing the plate and an opaque stencil with 

 some design punched through it. Place this plate at any distance 

 from a known source of light. Give any exposure to the plate. 

 If the plate be now developed at a considerable distance from 

 the lamp, a negative image will appear. The lamp should be 

 vertically over the developing bath. If the bath in a similar 

 exposure is taken nearer to the lamp, the negative will be a 

 poorer one. At a still nearer distance, nothing will develop. 

 In the parlance of the photographer, the plate will fog. This is 

 called a zero plate. It the plate be developed at a point still 

 nearer the lamp, the picture will appear as a positive. The con- 

 ditions of zero plate as a function of exposure is now being 

 studied. A given exposure is a time integral of the effect due to 

 any given illumination, at the point of exposure. 



The paper referred to gives half-tone reproductions of photo- 

 graphs thus developed in the light as positives. In one an 

 attempt was made to " fog " a plate into a zero condition before 

 it was put into the camera for exposure to a street scene. The 

 plate was fogged by X-rays for two hours, and was exposed in 

 the camera for ten minutes. It was developed in a hydrochinone 

 bath, within one foot of a sixteen-candle lamp. The result is a 

 superb positive of the street scene in which the moving objects 

 leave no trace. The plate was an instantaneous isochromatic 

 plate by Cramer. 



Another picture reproduced was a picture on the same kind of 

 plate, obtained by an exposure of one minute in the camera. The 



* No. 6, vol. x, Transactions. 



