562 REPORT— 1901. 



If now we drop the heavy sphere of lead we shall have the sudden variation 

 required for the experiment. 



If my idea is right we shall observe at that same moment an impulsion of the 

 spot of light on the scale in the direction of the motion — that is to say, in a direc- 

 tion contrary to that which would he observed if the assumed eil'ect does not exist. 



The apparatus for making the experiment is now ready, and I hope to obtain 

 results before long. 



I will point out that astronomical observations caimot answer this ques- 

 tion because in the motions of the planets there are only very small changes 

 of the ' flux of gi'avitation ' ; and, besides, the distances are enormous. Moreover, 

 as these changes would be always reversed in the course of a complete 

 revolution, their very small effects woidd neutralise one another. 



6. A Discussion on the proposed New Unit of Pressure, opened hy a Paper 

 hy Dr. C. E. Guillaume. — For Dr. Guillaume's Paper see Reports, 

 P- 'J'l- 



7. The MicMson-Morley Effect. By W. M. Hicks, F.R.S, 



In the theory of this experiment, as usually presented, no account is taken of 

 the alteration in wave length produced by reflection from a moving surface, nor 

 of the alteration in the direction of incidence as the drift alters, when the source 

 of light is fixed to the apparatus. When this is done it follows that the pheno- 

 mena to be expected are not precisely the same as those usually supposed, and in 

 certain cases the displacement of the fringes is subject to a quite different law. 

 The two sets of interfering waves, when there is drift, have not the same wave 

 length in space, although their aiipavent frequencies at any point moving with the 

 apparatus are equal. Consequently interference fringes are produced on a screen 

 which is fixed to the apparatus, and these fringes are displaced a certain number of 

 bauds when the apparatus drifts. Usually, however, the fringe is observed by an 

 optical apparatus which produces an image on the retina. But the two inter- 

 fering pencils from any point of the actual fringe, when they arrive at the retina, 

 have a different phase-difference from that at the original point Consequently 

 the image of the central bright line will not itself be a bright line. The central bright 

 band on the retina will be the optical image of another point on the original, and 

 the fringe-image shows the original one displaced by a certain amount which 

 alters with the drift. The observed displacement is therefore the resultant of 

 two others, one of which may in certain circumstances quite mask the other. 

 Supposing the drift of the apparatus to be comparable with that of the earth's 

 orbital motion — say 10~^ times that of light — it was shown to be possible that 

 in Michelson's actual experiment the arrangements were such that the effect he 

 expected was quite masked by the other. 



8. The Laio of Radiation. By Dr. J. Larmor, F.E.S. 



9. Radiation of Heat and Light from a Heated Solid Body. 

 By Dr. J. T. Bottomley, F.R.S. 



In this paper an account is given of recent experiments on radiation of 

 heat and light from a heated solid body commencing with the very lowest 

 temperature at which a heated body becomes visible and proceeding to tem- 

 peratures approaching white heat. The experiments were made on pairs of 

 platinum strips specially prepared for the author by Messrs. Johnson & Matthey. 

 The strips were 1^ mm. broad, and extremely thin. One of each pair was 

 highly polished, and the other was coated with lampblack. The pairs of strips 



