in Theory and Practice. 



379 



accuracy is wished and the overlapping- spectra of different 

 orders are to be used, that this imperfection must be guarded 

 against. Since a 10,000 grating has on the whole better 

 definition than a 20,000 one, and as it is much cheaper, it is 

 better to use one in all cases when possible. For use with the 

 micrometer- eyepiecCj when of course the ultra-violet spectra 

 do not interfere, one can always be used. 



It is only when work is to be done with the camera in the 

 ultra-violet part of the spectrum that it becomes necessary to 

 use a 20,000 grating. This is due to the fact that, for the 

 same dispersion with a 20,000 grating as for a 10,000, there 

 are fewer overlapping spectra. The range of concave gratings 

 mounted as above are as follows : — 



Lines 

 per inch. 



First 

 Spectrum. 



Second 

 Spectrum. 



Third 

 Spectrum. 



Fourth 

 Spectrum. 



Fifth 

 Spectrum. 



10,000 

 14,438 

 20,000 



Entire. 

 Entire. 

 Entire. 



Entire. 



Entire. 



To 6,000 



Entire. 

 To 5,760 

 To 4,000 



To 6,000 

 To 4,330 

 To 3,000 



To 4,800 

 To 3,460 

 To 2,400 



These limits are taken at the centime of the photographic 

 plate. At the end of the plate the limit is somewhat greater, 

 being 6260 in the 2nd spectrum for a 20,000 grating. 



With a grating of 21*5 ft. radius, the width of the spectrum 

 varies from ^ in. to 4 in. In the green of the 1st spectrum 

 of a 20,000 grating it is f in., and in the green of the second 

 it is 2f in. This gives an idea as to the width of the photo- 

 graphic plate which is required. 



The scale of the negatives in the various spectra, with 

 gratings of 21*5 ft. radius, is as follows : — 





o 



Scale of Spectra as compared with Angstrom's Map. 



Lines per inch. 



1st. 2nd. 



3rd. 



4th. 



10,000 

 14,438 

 20,000 



•26 

 •37 

 •52 



•51 



•75 

 103 



•77 

 1-12 

 1-55 



103 

 1-50 

 207 



i. e. using a 20,000 grating in the 3rd spectrum, the scale is 

 1*.55. This means that T. 5 5 millim, on the photographic plate 



