328 



Anniversary Meeting. 



[Nov. 30, 



The subject of vibrations naturally leads on to a mention of other 

 hydro- dynamical researches. Lord Rayleigh has investigated the 

 motion of waves of finite height, and in particular has shown that the 

 "great solitary wave" of our late Fellow, Mr. Scott Russell, has a 

 determinate character ; and he has investigated the circumstances of 

 its motion to an order of approximation sufficient to apply to waves of 

 considerable height. 



Lord Rayleigh has examined more fully than had previously been 

 done the theory of diffraction gratings, and the effects of irregularities ; 

 and also investigated the defining power of optical combinations, and 

 its limitation by diffraction and spherical aberration. 



He has lately been engaged in the elaborate re-determination of 

 the B.A. unit of electrical resistance. 



The Rumford Medal has been awarded to Captain W. de W. Abney, 

 R.E., F.R.S. Captain Abney has contributed largely to the advance- 

 ment of the theory and practice of photography by numerous investi- 

 gations. In the Bakerian Lecture for 1880, he has given an account 

 of a method by which photography can be extended to the invisible 

 region below A, which had been hitherto but very imperfectly examined 

 by means of the thermopile. 



Making use of plates prepared with silver bromide in a particular 

 molecular condition, Captain Abney, by means of a diffraction grating 

 containing 17,600 lines to the inch, constructed a detailed map of the 

 infra-red region of the solar spectrum extending from A down to 

 X 10,650 (Plate XXXI, "Phil. Trans.," 1880). The lowest limit of 

 this map was fixed by conditions of the diffraction apparatus, and not 

 by a falling-off of the sensitiveness of the plates at this low point ; 

 for, when a prismatic apparatus was used, photographs were obtained 

 which show a continuous spectrum down as far as X 1*2,000. 



In a subsequent paper ("Phil. Trans.," 1881, p. 887), Captain 

 Abney, working with Lieut. -Col. Festing, R.E., applied this new 

 extension of photography to a research on the influence of the atomic 

 grouping in the molecules of organic bodies on their absorption in the 

 infra-red region of the spectrum. The authors believe that their 

 results indicate, without much doubt, that the complex substances 

 they examined can be grouped according to their absorption spectra, 

 and that such grouping, as far as their experiments go, agrees on the 

 whole with that adopted by chemists. They have more confidence in 

 their results, as they were careful to select such bodies as might 

 be regarded as typical : but, of course, much patient labour of many, 

 for a long period, will be necessary before this new branch of physico- 

 chemical research can be regarded as fully established in any complete 

 form. 



Captain Abney has since carried on his work in this new region of 



