198 



NA rURE 



[June 30, 1904 



LETTERS TO THE EDITOR. 



[T/je Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is talien of anonymous communications.^ 



Variation of Atmospheric Absorption. 



REtERRiXG to a communication from me in Nature of 

 November 5, 1903 (vol. Ixix. p. 5), to the effect that a 

 definitely less amount of heat had been received at the 

 earth's surface in the last years than in preceding ones, 1 

 would ask attention to an article published in June in the 

 Astrophysical Journal, in which I have further indicated 

 that the diminution of the heat received in 1903 may well 

 have been due not only to an increased absorption in our 

 own atmosphere, but in part to a real change in the solar 

 emission, connected with a diminished transmissibility of 

 the solar envelope. 



Still more recently I have made further experiments, as 

 yet unpublished, on changes in the transmissibility of the 

 t-olar atmosphere. These experiments, made by the study 

 of homogeneous ra\s from a large solar image formed bv 

 a horizontal telescope of 140 feet focus, are independent of 

 changes in the earth's atmosphere, and have indicated that 

 the absorption in the solar envelope has decreased within 

 •the last six months. 



In agreement with this, independent computations of the 

 total solar radiation through our own atmosphere (and so 

 far less trustworthy than those just mentioned) tend to 

 show that the radiation of the bun has somewhat increased 

 during the same interval. I desire not to be understood as 

 stating that these recent changes have undoubtedly occurred, 

 but I feel that there is increasing probability of the con- 

 firmation of this result. 



There is no novelty in the suggestion th.-it there may be 

 ■an increase or diminution of solar heat and light due to 

 •various causes, and since my earliest staten:ent of the 

 absorption of the solar atmosphere, in the Comptcs rendus 

 of the Paris .■\cademy of Sciences for March 22, March 

 29, and September b, 1875, the subject has, in fact, 

 engaged my continued attention. 



What I wish to remark now is that it is only in com- 

 paratively recent years that the gradual perfection of the 

 ijolometer and other apparatus is providing specific data 

 which render it likely that such changes are now coming 

 within our means of direct recognition. 



In sum, the result of the most recent spectrobolometric 

 observations is an increasing probability that the solar 

 radiation itself varies in a degree appreciable to our present 

 means of daily observation, and a strengthening of the 

 belief I have elsewhere expressed that it probably varied 

 through much larger ranges in the past, and may do 

 so again in the future 



It will be seen that I do not venture yet to assert without 

 restriction conclusions like these, which, so far as they 

 may be shown to be true, are not merely of abstract interest, 

 but which in a utilitarian sense may be said to be of far- 

 reaching concern ; yet I think it time to ask more general 

 attention to them. S. P. Langlev. 



Smithsonian Astrophysical Observatory, June 20. 



Use of Radium in Section Cutting. 



Everyone who has to cut microtome-sections of material 

 embedded in paraffin-wa.\ is frequently troubled by the 

 electrification of the sections. The electrification causes the 

 sections to adhere to the microtome-knife and to fold on 

 themselves instead of being pushed easily across the blade 

 of the knife so as to form a smooth ribbon. The adhesion 

 to the knife also renders the transference to the microslip 

 difficult, and often leads to the breaking up of sections and 

 consequent loss of continuity in the seriation. Further, 

 •even when successfully detached from the knife, the 



NO. 1809, VOL. 70] 



electrified sections are apt to fiy about in such an erratic 

 way that it is often a matter of difficulty to arrange them 

 in an orderly manner on the slip. 



These undesirable phenomena may be completely avoided 

 by fixing a 5 mg. tube of radium bromide on the micro- 

 tume-knife close to where the paratTm ribbon is forming. 

 Apparently the radiations from the radium discharge the 

 electrification of the paraffin sections by ionising the air in 

 their neighbourhood. Henry H. Dixon. 



Botanical Laboratory, Trinity College, Dublin. 



The Blondlot ;?-Rays. 



There is reason to think that M. Blondlot has rendered 

 very valuable service to science by directing the attention 

 of physicists to the remarkable, if not altogether mysterious, 

 class of phenomena with which he has recently had to 

 deal. 



There can be no doubt that the phenomena, strange as 

 they may seem to be, which he and his colleagues have 

 observed, whether appearances or realities, or, should I 

 say, subjective or objective effects, still leave something 

 that remains unexplained. Psycho-physiological pheno- 

 mena are not the less interesting because they happen not to 

 be physical effects as ordinarily understood, and if they 

 can lead scores of trained physicists astray, they should be 

 regarded as all the more important. 



For one, I am unhappy in that I cannot merely not see 

 the effects, but neither have I been fortunate enough to 

 meet with anybody who, on severe cross-examination in 

 the dark, did not satisfy me that the variations in bright- 

 ness which he had observed were altogether subjective, and 

 the result of imagination or fatigue, for expectation counts 

 for a great deal in these observations, and concentration of 

 attention for still more. 



The one thing that seemed conclusive about these rays 

 was that they produced so great an increase in the bright- 

 ness of a small spark that the effect could be photographed, 

 and M. Blondlot has himself shown us photographs which 

 it would appear show unmistakably this result. 



I have followed in his footsteps as closely as I could, but 

 unfortunately have not obtained any difference in the photo- 

 graphic effects which could not be attributed to a spurious 

 cause. In M. Blondlot's experiment there is no proof that 

 the diminished brightness of the spark, when a lead screen 

 is interposed, is not due to the presence of the metallic 

 screen itself, which is so close to the spark that it would 

 damp the oscillations of the spark and affect its photo- 

 graphic effect. I have preferred to put out the source of 

 H-ravs altogether, and to wait for some time, ten minutes or 

 so, or to place a lead screen at a considerable distance from 

 the spark. 



I have used a spark of about i/ioth mm. between two 

 brass spheres, each of about i cm. radius. The effect on a 

 photographic plate 2 cm. away is that of a luminous band 

 the edges of which are close to the spark, practically straight 

 lines, and at a greater distance curve round, being branches 

 of two hyperbolas. 



A change in the brightness of the spark is accompanied 

 by a broadening of the band, and a change in the intensity 

 of the diffuseness of the plate. The breadth of the band 

 depends upon the exposure, and conversely upon the bright- 

 ness of the spark. Except when there were errors in the 

 adjustment of the apparatus, the two photographs taken on 

 the same plate indicated the same brightness. 



It is interesting to note in connection with this point 

 that M. Jean Becquerel maintains that the alleged change 

 in brightness of a phosphorescent screen is really due to an 

 effect on the retina due to the n-rays which are reflected 

 by the luminous body. This explanation, however, will 

 not fit in with M. Blondlot's photographic effects, as these 

 rays are not supposed to produce any direct photographic 

 effects. But M. Becquerel's conclusion confirms my result 

 that the n-rays, if there be any such, do not really intensify 

 the brightness of a luminous body, even if this be the 

 property by which they were supposed to have been dis- 

 covered. John Butler Burke. 



Cavendish Laboratory, June 21. 



