June 4, 1896] 



NATURE 



100 



days. The coordinites of the light 

 as follows : — 



Days. 

 O'OO 



o'05 

 010 

 0-15 



0-20 



Mag. 

 1350 

 13-87 

 1435 

 1470 

 1472 



0-35 



040 



0-45 



It thus appears that the star remains about minimum brightness 

 during half the period, while the maximum luminosity is of 

 relatively short duration ; the decrease in light is rapid, and the 

 rate of increase still more rapid. The succession of changes 

 does not seem to correspond with those of any previously known 

 class of variable stars. 



RECENT RESEARCHES ON RONTGEN RA YS. 

 'X'HF; novelty of Prof. Kontgen's skeletal photographs has 

 ■'■ almost worn ofi", and the field of research opened up by his 

 observations is now mainly occupied by scientific workers, who 

 are endeavouring to analyse the rays, and to extend the know- 

 ledge of their characteristics, rather than to produce startling 

 pictures capable of exciting the wonder of the general public. But 

 though the interest of scientific dilettantes has waned, the in- 

 vestigators who remain in the field are still so numerous that it is 

 hardly possible to keep in touch with the multitude of observa- 

 tions published ; and published in some cases, perhaps, a little 

 prematurely. A number of interesting results have been re- 

 corded from time to time among our "Notes"; but so many 

 papers and communications have been received during the past 

 Jew days, that they are now brought together for readier refer- 

 ence, as has been done in several previous issues of Nature. 



Attempt to Polarise Ronlgen Rays. 



Ur. John Macintyre, whose observations on the capabilities 

 of Riintgen rays have formed the subject of several letters and 

 notes in these columns, has sent us an account of an attempt to 

 polarise the rays. Different views have been expressed about 

 the possibility of polarising the rays by means of tourmalines, 

 and although Dr. Macintyre's experiments seem to indicate a 

 negative result, they are of such importance that they deserve to 

 be put on record. 



The source of electricity was the main, and the measurements 

 across the terminals (with Lord Kelvin's cell-tester and ampere 

 gauge) were lo volts and lo amperes. The spark of the Rhum- 

 korft" coil was 6 inches, and a mercury interrupter was used. An 

 ordinary Crookes' focus tube, enclosed in cardboard to exclude 

 all light, was excited by the above, and the vacuum carefully 

 arranged to give the maximum fluorescence by gently heating 

 the bulb with a spirit-lamp. Screens of barium platino-cyanide, 

 potassium platino-cyanide, and lithiumrubidium-platino cyanide 

 were tried. The two tourmalines were got as nearly alike as 

 possible, the measurements of each being : length, 47 mm. ; 

 breadth, 12 mm. ; thickness, 2 mm. ; and the experiments were 

 carried out in a dark room. 



In the first experiment, on placing one tourmaline between the 

 source of the Rontgen rays and the screen, and directly in con- 

 tact with the latter, a distinct shadow wa^ seen due to absorption 

 of the rays. On placing the second tourmaline parallel with the 

 first, a difference in density of the shadow was immediately ob- 

 served. When the tourmalines were gradually turned at right 

 angles to each other, a dark square area could be seen where the 

 two crossed. A source of error was, however, suggested in this 

 experiment. One ol the tourmalines could not be in as close 

 contact with the screen as the other ; and on account of the 

 manner in which the Rontgen rays pass from a point on the 

 platinum plate in such a Crookes' tube, difi"erences were ob- 

 served in the shadows of the four arms of the cross formed by 

 the tourmalines. For example, (i) if the horizontal tourmaline 

 were next to the screen, and the vertical one behind it, the two 

 arms above and below the square dark central area were less 

 sharply defined than the two arms on e.ach .side of it, and con- 

 sequently the shadows appeared to be ditTerent. (2) Although 

 on the square portion corresponding to where the tourmalines 

 crossed, one got a darker shadow still, it might only be due to 

 the difl'erence in thickness of the two layers. 



A second observation was then made. One of the tourmalines 

 was broken in two portions, and one of these was placed parallel 



NO. 1388, VOL. 54] 



with and the other perpendicular to the other tourmaline. Again 

 the dark square area was seen by direct vision. Dr. Macintyre 

 could not s.iy, however, that the density was greater than where 

 the other portion of the broken tourmaline was laying parallel 

 with the whole one. This rather suggested that the square dark 

 area was caused by difference of density only. In a third series 

 of observations photographs were taken with different exposures 

 —one with a single flash of the tube, due to one interrup- 

 tion of the coil ; others with much longer exposures, but in 

 all the same difficulties in distinguishing between the two con- 

 ditions arose. (Copies of these photographs have been received 

 from Dr. Macintyre. ) In the first photograph a shadow of one 

 tourmaline was obtained, proving the absorption of some of the 

 Rontgen rays. In the second photogiaph, of one whole 

 tourmaline and a portion of the other, a greater density can be 

 noted where two layers are lying parallel with each othei than 

 where only one tourmaline interferes with the rays. The third 

 photograph shows the unbroken tourmaline covered at one part 

 by a portion of the broken tourmaline lying parallel with its 

 axis. The other part of the broken tourmaline is placed at right 

 angles, and Dr. Macintyre raises the question whether the 

 density of the square area is greater than where the two 

 tourmalines are lying parallel with each other. In his opinion, 

 the photographs bear out the observations by direct vision, and 

 appear to give negative results ; and an examination of the two 

 photographs which form the result of his crucial experiment, 

 leads us to conclude that there is not any appreciable difl'erence 

 of brightness between them. 



Rontgen Rays and the Resistance of Selenium. 



Mr. J. W. Giltay, Delft, Holland, has sent us the following 

 important communication on the influence of Rontgen rays upon 

 the resistance of selenium. 



Some weeks ago, the possibility of Rontgen rays having an 

 influence on the resistance of selenium occurred to me. I 

 made a preliminary experiment to put this idea to the test, but, 

 probably owing to the poor state of my induction coil, I failed 

 to get any effect. Want of time prevented me from trying 

 again with another coil. 



I told my failure to Prof. H. Haga, of Groningen University, 

 who kindly undertook to investigate the subject. The selenium 

 cell I made for him was of the Shelford-Bidwell type (NATtiRE, 

 November 18, 1880), the working surface was 20 x 44 mm. 

 The resistance of this cell was in darkness 31,600 ohms, in 

 diffuse daylight it was about 15,300. 



Prof. Haga with this cell got the following results, which I 

 publish in this letter with his full approval. 



The Crookes' tube he used was of the ordinary pear form 

 (not a focus tube), and highly evacuated, giving undoubtedly a 

 very strong Rontgen eff-ect. The induction coil was one of 

 RuhmkorfTs, of a length of 60 cm. ; the battery for driving the 

 coil consisted of five accumulator cells. 



The distance between the selenium cell and the under part of 

 the tube was 3 cm. The eel was covered with pasteboard, and 

 over this was laid a thick sheet of zinc. The resistance of the 

 cell was njw measured by the bridge method, one dry cell 

 acting as the battery, contact being of course made only moment- 

 arily. The resistance in the dark was found to be 31,600, as I 

 remarked before. Now the induction coil was started and 

 worked during just one minute ; the resistance of the cell was 

 then immediately measured again, and found to be exactly the 

 same. This proved the wires carrying the induced currents 

 and the coil itself to have no influence on the cell. 



Now the zinc plate was removed and replaced by two thin 

 aluminium sheets (two instead of one, to prevent heat rays 

 falling on the cell). The coil was now worked during one 

 minute, and immediately after stopping it the resistance of the 

 cell was taken. This was now found to be 26,400. 



The resistance was not measured during the radiation, else it 

 would probably have been found to be a little less than 26,400, 

 but immediately after the coil having been stopped. The 

 measuring of the resistance took about one minute. _ After 

 having left the cell at rest during 20', the resistance had risen to 

 29,500 again. 



Prof. Haga made several experiments, always with the same 

 qualitative results. 



A simple kind of bolometer, consisting of strips of tinfoil 

 (11-85 ") '•i'l not ^l^o^^ any change of resistance by Rontgen 

 radiation. 



