1870.] Atmospheric Lines of the Solar Spectrum. 



3 



mator of the spectroscope had bent, by its own weight, at the flanges 

 where it unites with the drum ; and this deterioration involved remedies 

 for which the available means were but too limited. In this way the au- 

 tumn of 1868 passed away, to my great disappointment, without bringing 

 me any results save experience ; and I eventually left Mussoorie, as usual, 

 for the winter of 1868-69, to carry on my survey duties at Dehra. 



7. Narrative continued. — On my return to this place about the end of 

 April 1869, I found, as was to be expected, that, owing to the small amount 

 of rain in the winter, the hazy state of the atmosphere still continued ; so 

 that it was not until last October (1869) that I was able fairly to com- 

 mence work at sunset. Even then I had to discover by experiment that, 

 to observe the full atmospheric effect on the spectrum, I must command a 

 view of the sun down to the very horizon. To suit this new condition, I 

 had to shift the spectroscope twice to adjoining positions, and to modify 

 the map I had already prepared. 



8. Construction of spectroscope. — The spectroscope is made up of a 

 drum containing three flint-glass prisms, one of which may be made non- 

 effective at pleasure. The collimator unfortunately consists of three 

 pieces, of which the nearest portion is rigidly fixed to the drum. The 

 second piece unites with the drum piece by means of three screws driven 

 through flanges ; the third piece, which unscrews for packing, bears the 

 slit at its far end. The spectrum is viewed by a telescope provided with 

 four eye-pieces, whose magnifying-powers I determined with a dynameter 

 to be as follows : — 



Eye-piece. Power. 



No. 1 9 



No. 2 17 



No. 3 25 



No. 4 54 



A graduated circle and vernier, read by a side-telescope, provide the 

 means for measuring distances between the lines ; and the foregoing are 

 mounted equatorially on a tripod-stand. 



9. Details. — In observing the spectrum sun high, I was soon struck by 

 the difference in intensity of the lines as seen at Mussoorie and as given in 

 Kirchhoff's well-known map. Excepting the decided lines A, B, C, and D, 

 all the others appeared comparatively faint, and even wanting in defini- 

 tion. It is also worthy of remark, that while Kirchhoff used a power of 

 forty to view the spectrum with, I am unable to employ any higher power 

 than eye-piece No. 2 (power 17). My observations were thus made with 

 eye-piece No. 2, using all three prisms ; and this combination, the most 

 powerful at my command, produced an image only some three-tenths of 

 what Kirchhoff, I presume, actually saw. 



10. Details. — The comparative smallness of image presented to view 

 has necessarily added sensibly to the difficulties of the undertaking ; for, 



b 2 



