NA TURE 



[April 



8q6 



Traiisailions of the Inslitution for information upon this intricate 

 but extremely interesting subject. 



Captain Kriloffs paper was of a completely mathematical 

 character, and indeed was of far too abstruse a nature to follow 

 during the reading. It depended on an appendix of many pages 

 containing columns of figures which would require carefu study 

 to master. 



Mr. Little's paper was of practical interest to those concerned 

 in the carriage of grain. 



The proceedings were brought to a close by the usual votes of 

 thanks. 



The summer meeting this year is to be held in Hamburg 

 during the early part of June. Extensive preparations have 

 been made for the reception of members, and there is no doubt 

 the meeting will be of quite an international character. The 

 success of last year's meeting in Paris has encouraged the 

 Council to go abroad again. 



It may be added that the Institution is increasing in numbers 

 at a rapid rate, there being a greater addition to the roll of 

 membership at this meeting than has ever before taken place. 



RECENT WORK WITH RONTGEN RA YS. 

 SEVERAL important communications referring to work upon 

 ■^ Rontgen rays have come before our notice during the past 

 week. , While some experimenters are perfecting the methods 

 so as to develop the capabilities of the rays, others are investi- 

 gating the physical characteristics pertaining to them, and in 

 both directions of work clear advances have been made since 

 our last eclectic statement of the contributions to the subject 

 founded by Rontgen's discovery. 



Prof. Alfred M. Mayer, of the Stevens Institute of Technology, 

 has sent us the following account of experiments carried out 

 by him on the polarisation of Rontgen rays. 



"Of the remarkable properties of the Rontgen rays, the one 

 of the greatest interest is that these rays are not polarisable ; 

 for this property shows that these rays, unlike those of light, 

 are not propagated by vibrations transverse to the direction of 

 their progress. To decide conclusively this point certain pro- 

 perties, shown by Rontgen's experiments, must be possessed by 

 the substance which is to act on the Rontgen rays, viz. (i) a 

 low density ; (2) the substance must be very thin, and yet give 

 complete polarisation to transmitted light. These two proper- 

 ties are eminently peculiar to herapathite, an iodo-sulphate of 

 quinine. Its density is only i '8, and crystals of herapathite of 

 only 0*05 mm. in thickness, with their axes crossed at 90°, 

 entirely obstruct the incident light, so that their crossed portions 

 appear intensely black. 



" Six discs of glass, o'i5 mm. thick and 25 mm. in diameter, 

 were covered with crystal-plates of herapathite crossing one 

 another at various angles. Where they crossed at right angles 

 they gave a black field. These discs were fastened to the 

 surface of a screen of compressed brown paper, which was 

 found to be impervious to the actinic action of a powerful arc 

 light acting during two hours, and placed i foot in front of 

 the screen ; the latter covering a sensitive photographic plate. 

 On this screen were also placed three discs of the same glass, 

 overlapping one another, so that i, 2, and 3 thicknesses of the 

 glass had to be traversed by the X-rays before they reached the 

 photographic plate. These discs served as standards with 

 which to compare the action of the X-rays on the discs covered 

 with herapathite. On the same screen was also placed a square 

 of yellow blotting-paper, f mm. in thickness, having on its 

 surface superposed herapathite crystals from two to four layers 

 deep. 



" This screen so prepared, and covering a sensitive plate, was 

 exposed to the radiations of the Crookes' tube ; in the first 

 experiment for half an hour, in the second for one hour, and in 

 the third for two and a half hours. On developing these plates 

 there was not the slightest trace of the presence of the 

 herapathites. The photographs of the glass discs had not the 

 slightest mottling on their surfaces. Their surfaces appeared 

 throughout to the unaided eye, and also when examined with a 

 magnifying glass, with a uniform illumination and grain through- 

 out. The herapathite, of \\\e thickness used in these experi- 

 ments, does not appear to screen at all the X-rays ; for all the 

 discs carrying it appeared exactly alike, in illumination and in 

 grain, to the photograph of a similar disc having nothing on its 

 surface. But the action of the rays on the square of blotting- 



NO. T379, VOL. 53] 



paper carrying the herapathites showed this in a more con- 

 clusive manner ; for where this paper covered the photographic 

 plate nothing was visible, except by the most careful scrutiny, 

 and with the most favourable illumination, and then a mere 

 ghost of the paper could be detected, but with no traces what- 

 ever of the herapathites. 



" These experiments appear to have shown conclusively what 

 Rontgen found by his experiments ; viz. that the X-rays are not 

 polarised by passing through doubly- refracting media." 



At a recent meeting (March 3) of the Dublin University Ex- 

 perimental Science Association, Dr. J. Joly, F.R.S., described 

 experiments made by him on the " Lenard- Rontgen " rays. He 

 has found that the rays are reflected at the surface of mercury, 

 lead, glass and wood. A photographic plate was enclosed in 

 a light-tight carrier of millboard, upon the outside of which a 

 copper ring was attached. This was exposed in the geometrical 

 shadow of a thick lead plate to rays which entering a slot in 

 the plate were reflected at the surface of mercury. An ex- 

 posure of over an hour gave the shadowgraph of the ring. The 

 position of this upon the plate indicated that the rays had ap- 

 proached from the direction of the reflecting mercury surface. 

 Removing the dish of mercury, a much fainter image was ob- 

 tained apparently from rays reflected from the wood beneath. 



If the rays are received upon the carrier after passage between 

 two parallel lead plates, the dark band formed upon the sensi- 

 tive plate will be found to be bordered by heavy black lines. 

 This was traced to a very complete reflection at grazing inci- 

 dence to the lead plates, corresponding to the manner in which 

 light is reflected at grazing incidence. A photographic plate 

 exposed to light passing between the lead plates shows, in fact, 

 a similar dark border ; substituting glass plates for the lead, 

 similar eff"ects were obtained. This marked grazing reflection 

 rendered it possible to concentrate the rays to an imperfect focus 

 by causing them to pass through a conical tube of lead open at 

 both ends, when a strengthening of the effects was formed on 

 exposing at the narrow end of the cone. 



Before the Royal Society on March 19, Lord Blythswood de- 

 scribed some experiments which indicate that the X-rays can be 

 reflected. He placed a vacuum-tube, A (Figs. 1-2), behind a lead 

 screen, bb, 18 in. x 12 and \ thick. The screen had a 2in. hole 

 in it with a 2-in. pipe attached ; 4 in. from the vacuum-tube was 

 placed a speculum-metal mirror d, 4 in. x i\, at an angle of 45' 

 with the lead screen ; 4 in. from the mirror was a light-tight 

 zinc box, E, with aluminium window, F ; inside came first the 

 objects, G, stuck on to a black cardboard, H, then I, the photo- 

 graphic plate. The following objects were photographed in 

 about twenty minutes : (i) Some brass clock wheels. (2) A 

 screw-cutting gauge. (3) Two lead discs. (4) The mirrors, 

 being two pieces of speculum-metal used by Lord Blythswood 

 to divide upon. 



Two other papers were read before the Royal Society 

 at the same meeting. In one of these, Mr. R. Erskine Murray 

 described experiments made in the Cavendish Laboratory of the 

 University of Cambridge, at Prof. J. J. Thomson's suggestion, 

 in order to find whether the contact potential of a pair of plates 

 of different metals is in any way affected by the passage of the 

 Rontgen X-rays between the plates. 



The vacuum bulb and induction coil for the production of the 

 rays were enclosed in a box lined with metal, so that the plates 

 and the apparatus used in measuring their contact potential 

 difference should be screened from any direct electrical disturb- 

 ances. At one side of the box there was a circular hole of about 

 3 cm. in diameter. The vacuum bulb was placed just inside this 

 hole, and directed so that the rays should stream out through it 

 in a direction perpendicular to the side of the box. In some 

 experiments this hole was closed by a tinfoil screen, which 

 allowed a large proportion of the rays to pass out while .shutting 

 in ordinary electrical disturbances. The plates whose contact 

 potential difference was to be measured were placed at a short 

 distance outside the box, in such a position that the rays could 

 fall on them. 



To measure their contact potential, Mr. Murray used the null 

 method described by Lord Kelvin in his paper given to the 

 British Association in 1880. In this method the value of the 

 contact potential is found by measuring the amount of the counter 

 potential which has to be applied to the pair of plates to reduce 

 the potential difference between their opposing surfaces to zero. 

 The counter potential introduced to effect this annulment must 

 obviously be equal and opposite to their contact potential 

 difference. Plence the numerical value of the latter is simply 



