8 Lt.-Col. J. Waterhouse — On 'Rowland's Diffraction Gratings. [Jan. 



The red end, from D to A, is particularly interesting, especially 

 about sunset. I have already noticed considerable variations in the 

 number of lines that can be seen b^^p^ the D lines. During the 

 rains, 11 could generally be easily counted, but during the dry cold 

 weather we have just been having, 7 or 8 only can easily be distin- 

 guished. Two Sundays ago, however, the full number of 11 were 

 counted ; though fainter than in the rains. Once one has mastered the 

 instrument, it will probably not be difficult to take photographs showing 

 many of the more notable changes in this part of the spectrum at dif- 

 ferent times of the day and seasons of the year. I have already suc- 

 ceeded in obtaining fair photographs of the B and a groups and even 

 of A, with possible traces of lines below in the ultra-red, though this 

 is nothing new. 



The apparatus has been made up at the Mathematical Instrument 

 Office, and I am indebted to my colleague, Major C. Strahan, R. B. for 

 many useful hints, and to Mr. T. Marshall, Mathematical Instrument 

 maker, for his assistance in making it up. 



The large map of the diffraction spectrum I have placed on the 

 board has been brought out by Prof. Rowland, and has, I believe, been 

 made with a concave grating of about 21 feet focus. This gives an 

 enormous dispersion, far beyond the power of the most powerful pris- 

 matic spectroscopes. A great deal more can, of course, be seen by the 

 eye with such an instrument than can be photogi'aphed, but Professor 

 Rowland believes he has very nearly reached the limit of visibility. 

 This map has now become the standard of reference for the solar spec- 

 trum ; at present it only extends to A. 5795 and does not include the D 

 group and below. It is to be hoped that it will be possible to extend it 

 at least to A.* With my small grating I can see almost all that is shown 

 on the maps, using an eyepiece of fairly strong power — though I cannot 

 photograph anything like so much. 



I should be glad to show the working of these instruments to any 

 members of the Society who would like to see them. 



BabiJ Sarat Chandra Das exhibited a coloured drawing giving a 

 bird's eye view of Tashi-lhun-po (the second city in Tibet) the capital 

 of the Tashi-Lama, obtained as a present from the Political Agent of 

 the Dalai Lama at Peking. 



The Philological Secretary read the following letters : 



1. Letter from Babu Gaurdas Bysack intimating the presentation 



to the Asiatic Society of two rare Tibetan MSS. by Omrao Giri Gossain, 



the Mohunt of the Bhotebagan Temple, Ghoosery. 



* Since this was written a new edition of this map extended to B has been 

 announced. 



