Febeuaey 14, 1896.] 



SCIENCE. 



235 



shortening the time of exposure gave en- 

 couraging results . A shortening of the time 

 of exposure and the simplification of the ap- 

 paratus employed is very desirable in the 

 practical application of this wonderful dis- 

 covery. The prospects are that both will 

 soon be reached. It should be mentioned 

 in this connection that a Whimshurst or a 

 Holtz machine can very well be employed in 

 place of the somewhat complicated appara- 

 tus employed so far. These machines should 

 be used to charge a small Ley den jar and 

 discharge it between small spheres which 

 are at a suitable distance from each other. 

 The tinfoil coatings of the vacuum tube 

 should then be connected suitably to the 

 coatings of the jar. This arrangement is 

 much simpler than the one usually em- 

 ployed and will in all probability give just 

 as good results — perhaps even better, be- 

 cause a disruptive character of the dis- 

 charges seems to improve the results. This 

 method, however, is offered as a suggestion 

 only, since the writer could not procure a 

 statical machine in time to convince him- 

 self of the actual value of the suggested ar- 

 rangement. The practical applicability of 

 this method of photography to surgery seems 

 certain. M. I. Pupin. 



Columbia College, February 8, 1896. 



EXPERIMENTS ON THE X-RAYS. 



Experiments with Kontgen's newly de- 

 tected X-rays have been carried on during 

 the past few days in the Dartmouth physical 

 laboratory by Prof C. F. Emerson and the 

 writer, and some of the preliminary results 

 already obtained may be worth recording. 



Of four Crook.es tubes first tried, but one 

 emitted rays which (with the exposure 

 given) made a visible impression upon a 

 photographic plate protected from the or- 

 dinary luminous rays. This tube is 4.7 cm. 

 in diameter and is cylindrical for a length 

 of 16 cm., then tapering to a point. The 

 platinum electrodes are on opposite sides of 



the cylindrical surface and are about 5 cm. 

 apart. A phosphorescent plate is interposed- 

 obliquely between the electrodes. In ac- 

 tion the phosphorescent surface is bom^ 

 barded by the discharge from the negative 

 pole. We have thus far usually excited the- 

 tube by a current from an efficient induction; 

 coil, but a Holtz machine has served about 

 equally well. 



The first successful experiment gave^ 

 after 12 minutes of exposure, a picture of a 

 knife and scissors hung on the side (1 cm. 

 thick) of a whitewood box, within which 

 the photographic plate had been placed. 



Subsequently, the Crookes tube was sup- 

 ported horizontally, and the plate-holder 

 could then be laid upon the table and any 

 object interposed that was desired. No 

 camera was employed, and the slide of the 

 plate holder was not drawn, so that no ex- 

 posure to the ordinary luminous rays could 

 occur. 



A coin and key concealed between two 

 boards of total thickness, 24 mm., were 

 shown after an exposure of 11 minutes, the- 

 tube being 15 cm. above the plate. The- 

 power of transmitting the X-rays has been 

 tested for a number of substances. Silver 

 and gold seem to be the most opaque of the 

 metals yet tried, although aluminium trans- 

 mits poorly. Glass is more opaque than 

 brass, and less so than hard rubber. Cork 

 transmits better than any other substance- 

 examined. (See Plate 4, Fig. 1.) 



An attempt to refract the rays by a car- 

 bon disulphide prism was unsuccessful, and 

 they seemed to pass through a pair of 

 crossed tourmalines without difSculty. No 

 effect except that of the usual metallic ob- 

 struction was noted when the wire convey- 

 ing the primary current was passed over the- 

 plate, or when the alternate current of th& 

 house circuit was sent through a loop of in- 

 sulated wire resting on the plate holder. 



"With the tube 9 cm. above the plate an 

 exposure of 15 minutes clearly brought out 



