1889.] Li-Col. J. Waterhouse— On RoivlancFs Diffraction Gratings. 3 



who would be proposed as a candidate for election that evening, had 

 agreed to undertake the duties and the Council were glad to have 

 obtained his services. 



The President exhibited a medal presented to the Society by the 

 Corporation of the City of London in commemoration of the visit of 

 Indian Potentates and others to London in connection with the Colonial 

 and Indian Exhibition of 1886. 



The President exhibited two Rowland's diffraction gratings, one 

 plane, the other concave ; also a copy of Professor Rowland's photogra- 

 phed map of the solar spectrum from A. 5075 to A 3795 obtained with a 

 large concave grating, and said : — 



Although these gratings and photographs are now fairly well 

 known in Europe and America, they are, I believe, quite new in India, 

 certainly in Calcutta, and therefore may be of interest to the meeting. 

 There can be no doubt that Professor Rowland has by the invention 

 of the concave grating, the perfection to which he has brought the 

 machine used for ruling the gratings, and the liberality with which they 

 have been put at the disposal of spectroscopists at very reasonable 

 prices, given an enormous impetus to spectroscopic research, especially 

 as applied to the sun and to the spectra of the elements as obtained 

 by means of powerful electric currents. 



The two gratings before the meeting are ruled with 14,438 lines to 

 the inch, which is the number usually adopted by Professor Rowland, 

 though they are also made with more or less close ruling. 



The principle on which these gratings work is not quite easily 

 explained, but will be found in the standard works on optics and spec- 

 troscopy. It will suffice to note that when a strong ray of white light 

 is thrown upon one of these gratings, on either side, right and left of 

 the direct and bright white central reflection, a series of more or less 

 brilliantly coloured spectra will be seen, all with their violet ends in 

 the direction of the central white reflection, and overlapping each other 

 more and more as the distance from the central reflection increases. 

 If the light of the sun be passed through a fine slit parallel to the 

 lines of the grating and viewed with a telescope, somewhat in the same 

 way as when viewing a prismatic spectrum, the Fraunhofer lines of the 

 spectrum can be seen with very great perfection, especially in the less 

 refrangible regions of the yellow, orange and red rays, which appear in 

 the true proportions of their wave lengths, instead of being unduly 

 compressed as they are when viewed through prisms. 



The spectra nearest the direct central reflection are termed the 



