234 



SCIENCE. 



[N. S. Vol. III. No. 59. 



vacuum the longer must be the time of ex- 

 posure under otherwise identical condi- 

 tions. The tubes were used as electrodeless 

 tubes, that is, a tinfoil strip was wrapped 

 around each end of the tube, and then the 

 tinfoils were connected to a high tension 

 coil with disruptive spark gap and Leyden 

 jar. The vacuum discharge is, of course, 

 in such cases due to the condenser effect. 

 The writer's experiments lead to the conclu- 

 sion that quite as powerful effects can be 

 produced in this manner as with electrodes, 

 and it obviates the risk of spoiling the tube 

 by excessive heating of the platinum wires 

 carrying the electrode discs. It is well to 

 observe here that with electrodeless tubes 

 the glass under the tinfoil becomes very 

 hot indeed in quite a short time, when 

 powerful, rapid electric oscillations are em- 

 ployed. But on account of the large tinfoil 

 surface which is in contact with the outside 

 air the temperature of the tube never be- 

 comes dangerously high. Some of the re- 

 sults of the writer's experiments seem to be 

 of sufficient interest to deserve a brief men- 

 tion here. 



An under-exposed plate fails completely 

 to bring out the relative absorption of the 

 materials placed in the path of the rays. 

 For instance, the photograph of a cigar box 

 made of aluminium sheet about -^^ of an 

 inch thick and containing several opaque 

 objects will show no detail if the exposure 

 is too shoi't. All that we see is the contour 

 of the box, and the area bounded by this 

 contour is uniformly illuminated. With a 

 sufficiently long exposure the contour is 

 still strong, but the area enclosed by the 

 contour is scarcely visible and the objects 

 in the box appear in sharp outline. Various 

 objects were photographed and the results 

 were similar to those obtained by Professors 

 Trowbridge and Wright. The most inter- 

 esting photograph obtained was that of a 

 pair of spectacles in a leather case (pee Plate 

 III., Fig. 1). It bears upon the subject men- 



tioned in the last paragraph. The exposure 

 lasted an hour; the tube had the highest 

 vacuum among the several tubes employed. 

 In all previous photographs the lenses of 

 these sfjectacles appeared as perfectly flat 

 discs of high opacity. In this photograph, 

 however, obtained with long exposure, the 

 varying thickness of the lens is beautifully 

 marked in the negative. The central part 

 of the lens is darkest, and then the increase 

 in luminosity toward the edge was gradual, 

 showing distinctly the curvature of the lens. 

 This photograph seems to support the 

 writer's belief that the relative amounts of 

 absorption in the various parts of tlie ob- 

 ject photographed are brought out in a 

 photographic plate if it is exposed a suffi- 

 ciently long time, but not otherwise. In 

 the photography of the human hand, for in- 

 stance, there is no trace of the skeleton un- 

 less the exposure is sufficientlj' long. The 

 contour surrounding the uniformly illumin- 

 ated surface of the hand is very easily ob- 

 tained with a short exposure. But to ob- 

 tain an image of the skeleton of the hand 

 the exposure must be sufficiently long, and 

 it appears that the longer the exposure the 

 stronger is the impression of the contour of 

 the bones and the weaker is that of the sur- 

 rounding flesh. 



Prof. Rontgen's photograph of the human 

 hand is the only one in which the flesh is 

 almost entirelj' invisible. In Mr. Swinton's 

 photograph the fleshj^ part of the hand is 

 nearly as strongly marked out as the bony 

 part. The writer cannot agree with Mr. 

 Swinton's opinion that this is due to over- 

 exposure, and prefers to consider the pres- 

 ence of the fleshy part of the hand as due 

 to underexposure. At any rate, the differ- 

 ence between the Kontgen photograph of 

 the human hand and the photographs ob- 

 tained by other experimentalists, including 

 the writer, seems to deserve further investi- 

 gation. A fluorescent screen placed in front 

 of the sensitive plate for the purpose of 



