January 23. 1896J 



NA TURE 



277 



O-ookes tube, which was excited for some minutes. On 

 development, shadows of all the articles placed on the 

 slide were clearly visible, some being more opaque than 

 others. Further experiments were tried with thin plates 

 of aluminium or of black vulcanised fibre interposed 

 between the objects to be photographed and the sensitive 

 surface, this thin plate being used in place of the wood of 

 the camera back. In this manner sharper shadow pictures 

 were obtained. While most thick metal sheets appear to 

 be entirely opaque to the radiations, aluminium appears to 

 be relatively transparent. Ebonite, 

 vulcanised fibre, carbon, wood, card- 

 board, leather and slate are all very 

 transparent, while, on the other 

 hand, glass is exceedingly opaque. 

 Thin metal foils are moderately 

 opaque, but not altogether so. 



As tending to the view that the 

 radiations are more akin to ultra- 

 violet than to infra-red light, it 

 may be mentioned that a solution 

 of alum in water is distinctly more 

 transparent to them than a solution 

 of iodine in bisulphide of carbon. 



So far as our own experiments go, 

 it appears that, at any rate without 

 very long exposures, a sufficiently 

 active excitation of the Crookes 

 tube is not obtained by direct con- 

 nection to an ordinary Rhumkortt 

 induction coil, even of a large size. 

 So-called high frequency currents, 

 however, appear to give good 

 results, and our own experiments 

 have been made with the tube 

 excited by current obtained from 

 the secondary circuit of a Tesla 

 oil coil, through the primary of 

 which were continuously discharged 

 twelve half-gallon Leyden jars, 

 charged by an alternating current 

 of about 20,000 volts pressure, 

 produced by a transformer with a 

 spark-gap across its high-pressure 

 terminals. 



For obtaining shadow photo- 

 graphs of inanimate objects, and 

 for testing the relative transparency 

 of different substances, the parti- 

 cular form of Crookes tube em- 

 ployed does not appear to greatly 

 signify, though some forms are, we 

 find, better than others. When, 

 however, the human hand is to be 

 photographed, and it is important 

 to obtain sharp shadows of the 

 bones, the particular form of tube 

 used and its position relative to 

 the hand and sensitive plate appear 

 to be of great importance. So far. 

 owing to the frequent destruction 

 of the tubes, due to overheating of 

 the terminals, we have not been able 



to ascertain exactly the best form and arrangement for this 

 purpose, except that it appears desirable that the electrodes 

 in the tube should consist of flat and not curved plates, 

 and that these plates should be of small dimensions. 



The accompanying photograph of a living human hand 

 (Fig. i) was exposed for twenty minutes through an 

 aluminium sheet "0075 in thickness, the Crookes tube, 

 which was one of the kind containing some white 

 phosphorescent material (probably sulphide of barium), 

 being held vertically upside down, with its lowest point 

 about two inches above the centre of the hand. 



NO. 1369, VOL. 53] 



By substituting a thin sheet of black vulcanised fibre 

 for the aluminium plate, we have since been able to reduce 

 the exposure required to four minutes. Indeed with the 

 aluminium plate, the twenty minutes' exposure appears 

 to have been longer than was necessary. Further, 

 having regard to the great opacity of glass, it seems 

 probable that where ordinary Crookes tubes are em- 

 ployed, a large proportion of the active radiations must 

 be absorbed by the glass of the tube itself If this is 

 so, by the employment of a tube partly constructed of 



Fig. I. — Photograph of a living human hand. 



aluminium, as used by Lenard, the necessary length of 

 exposure could be much reduced. 



-A. A. C. SWINTON. 



NOTES. 



At their scientific meeting on March 3, the Zoological 

 Society propose to discuss the much-vexed question of zoological 

 nomenclature. This subject will be introduced by Mr. Sclater, 

 the Secretary of the Society, who will read a pajier on the 

 "Rules for naming Animals," lately adopted by the German 



