May 7, 1896] 



NATURE 



the not rigorously plane surface of the photographic 

 fihii, would be unsuitable ; and the plan, which Lord 

 Rayleigh described in the discussion, of forming the film 

 on a solid metallic mirror, might be substituted for it. 



AH things considered, it seems not improbable that 

 Lippmann's process may be applied successfully to 

 X-rays at nearly grazing incidences on metallic mirrors, 

 and possibly even on non-metallic mirrors. 



Suppose now, for instance, the directions of the 

 incident and reflected rays to be inclined to the mirror 

 at angles of 'i of a radian (5"7). The distance between 

 the pLines of stratification in the photograph would be 

 ten times that which would be produced by the same 

 light at normal incidence. Thus if for example, the 

 wave-length of the particular X-light used is 5 x lo'^cms. 

 (or one-tenth of that of green light), the photograph 

 would show tints of from green to violet when viewed 

 nornxally, or at less or more oblique angles, by Lipp- 

 mann's ordinary arrangements. 



It is quite possible, however, that when we know some- 

 thing of the composition of Rontgen light, we may find 

 such great differences of wave-lengths ' in it, and so 

 much difficulty to obtain appro.ximately homogeneous 

 X-light by sifting through metal plates (as we sift 

 ordinary visible light by coloured glasses), or by other 



may be normally, according to Prof Lippmann's ordinary 

 procedure, will be seen as a complete spectrum in con- 

 centric circles, with violet in the centre, and red, of wave- 

 length 7'i5 X 10"", at the circle of 56° incidence ; but, if 

 viewed by an eye placed at the position of the source of 

 the violet light which photographed it, it will, according 

 to the principles explained by Dr. Lippmann in his paper, 

 be seen of uniform violet light throughout its whole area. 



Kelvin. 



THE OBSERVATORY AT MONT MOUNTER. 



THERE is no end to the generosity of M. Bis- 

 choffsheim. Not so very many years ago he 

 endowed science with an observatory at Nice, and now 

 again he has presented another, and this one is at the 

 high altitude of over 8900 feet The observatory is 

 situated on the summit of Mont Mounier, one of the 

 peaks in the Maritime Alps. The advisability of having 

 it at this spot was suggested by M. Bischoffsheim himself. 

 It was not till early in 1S93 that the plans were worked 

 out, but the observatory was sufficiently finished in August 

 of the same year, to allow observations of Venus to be 

 made before the planet passed into its inferior conjunction. 



■ 8900 feet). 



means if other means can be found, that the experiment | 

 which I have suggested may fail on account of want of 

 homogeneousness of the incident light. 



But here, suggested to me by thinking of oblique in- 

 cidence for the photographic light, is an illustrative 

 experiment which (with variations of detail to facilitate 

 realisation) cannot fail if Prof Lippmann will think it 

 worth while to try it. Place a point source of homogeneous 

 violet light (wave-length 4 x lo"-' cms.i so near to the 

 centre of the mirror and sensitive film that rays shall be 

 received at all angles of incidence from zero up to 56' 

 (being the angle of which the secant is 1 788). The thick- 

 ness of each stratum will vary in different parts of the photo- 

 graph in simple proportion to the secant of the angle of 

 incidence, and in the centre it will be equal to the half 

 wa^e-length. It will therefore vary from 2 x 10"^ in the 

 centre to 3'6 x lo""^ at the circle of 56 incidence. This 

 photograph, viewed or thrown on a screen as nearly as 



* It is to be hoped however that, very soon, we shall have definite 

 knowledge of wave-lengths of RSntgcn Xlight by diftraction fringes actually 

 seen instead of estimates of their smallness fromditTraction fringes not seen. 

 I should explain thai I am writing on the supposition which seems to me, after 

 iinich correspondence with Sir George Stokes, to t)e exceedingly probable 

 tlKit RAntgen light is merely ordinary transverst-vibrational light of very 

 >hort pcriotl. That its period is less than one-fifth that of green light seems 

 well proved by the skilful experiments described by Perrin in Comfites 

 rftiJiu, January 27, 1896, p. 187 ; and by Sagnac, Comptes icndns, March 

 10. P- 783- 



The buildings consist of a house for the astronomer 

 and his assistant, the actual observatory, which has a 

 revolving metallic dome (26 feet in diameter), and a 

 wooden hut, used as workshop or depot. The house and 

 actual observatory are united by a passage, which is 

 indeed a necessary arrangement, on account of the very 

 severe weather, and the snow, which sometimes lies thickly 

 on the ground. 



The observatory is a branch of the one at Nice, and at 

 the time that important observations were being made 

 at Nice, for the purpose of verifying M. Schiaparelli's 

 discoveries on the rotation of the planet Venus, they were 

 simultaneously being carried on at Mont Mounier by M. 

 Perrotin, and with most successful results. 



M. Bischoffsheim suggested that the observatory should 

 be a meteorological station ; it has therefore been 

 furnished with Richard's recorders, and instruments for 

 ascertaining the temperature, pressure, and other 

 conditions of the air. 



Nor is the observatory now isolated. For some weeks 

 the house has been connected by telephone to Beuil, the 

 nearest village with a telegraph office, a distance of five 

 miles. This was also done at the expense of M. 

 Bischoffsheim. It will therefore be possible to send 

 daily reports to the central meteorological office of the 

 observations made on Mont Mounier. 



NO. 1384, VOL. 54] 



