392 Royal Society .—The Hon. J. W. Strutt on the 



the Atlantic, and the north-easterly and north-westerly, very much 

 drier, which succeed the passage of the minimum. By the precipi- 

 tation of the vapour of the south-westerly winds a new area of pre- 

 cipitation is formed to the eastward of the initial area, accompanied 

 by its baric minimum ; and thus an apparent travelHng of the minimum 

 and its accompanying baric depression is induced, the direction 

 varying with the season ; it is also greatly modified by the general dis- 

 tribution of land and water. These are the principal features of the 

 work before us, which we strongly recommend to meteorologists as 

 indicating a course of study which in our opinion must contribute to 

 the advancement of Meteorology. While the Meteorological Com- 

 mittee of the Royal Society specifies in the maps issued under its 

 auspices the " baric depressions " which pass over Ireland, Great 

 Britain, and the north-west of Europe, and so far brings an important 

 meteorological phenomenon into view, we think that an extension of 

 Mr. Ley's work by the Office — in classifying for each month in 

 the year the paths of these depressions, and showing to a greater 

 extent than he has done their relations to season and also to the 

 configuration of land and water — would greatly contribute to the 

 improvement of the data on which storm-signals are ordered to be 

 hoisted, and would result in a greater percentage of the justified 

 orders than obtain at present. 



XL VII. Pi^oceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 316.] 

 June 20, 1872. — Sir James Paget, Bart., D.C.L., Vice-President, in 



the Chair. 

 ''pHE following communication was read : — 



-*■ " Preliminary Note on the Reproduction of Diffraction-gratings 

 by means of Photography." By the Hon. J. W. Strutt, M.xl. 



During the last autumn and winter I was much engaged with 

 experiments on the reproduction of gratings by means of photo- 

 graphy, and met with a considerable degree of success. A severe 

 illness has prevented my pursuing the subject for some months, and 

 my results are in consequence still far from complete ; but as I may 

 not be able immediately to resume my experiments, I think it desi- 

 rable to lay this preliminary note before the Royal Society, reserving 

 the details and some theoretical work connected with the subject for 

 another opportmiity. 



It is some years since the idea first occurred to me of taking ad- 

 vantage of the minute delineating power of photography to reproduce 

 with facility the work of so much time and trouble. I thought of con- 

 structing a" grating on a comparatively large scale, and afterwards 

 reducing by the lens and camera to the required fineness. I am 

 now rather inclined to think that nothing would be gained by this 

 course, that the construction of a grating of a given number of lines 

 and with a given accuracy would not be greatly facilitated by enlar- 

 ging the scale, and that it is doubtful whether photographic or other 

 lenses are capable of the work that would be required of them. 



