TABLE OF STANDARD WAVE-LENGTHS OF THE SPECTRAL LINES 561 



PHOTOGRAPHIC COINCIDENCES 

 CONCAVE, GRATING 10,000 LINES TO THE INCH 



20 



5853 



6569 



3024 " 3267 



Plates 7, 14 and 19 were imperfect, owing to clouds passing over the 

 sun, although a part (3218 to 3318) of Plate 14 was used for interpola- 

 tion, as observations were scanty in that region. 



It is seen that some of the plates have only one standard upon them. 

 With a plane grating it would be impossible to work them up, but with 

 the normal spectrum produced by the concave grating only one is 

 necessary, as the multiplier to reduce readings to wave-lengths is nearly 

 a constant. In working up a whole series of plates, there is no trouble 

 in giving a proper value to the constant for any plate in the series 

 which has only one standard. 



Plate 17 was measured twice by two dividing engines, and as it was 

 a specially good plate, each measure was given a weight equal to one 

 of the other plates. The principal error to be feared in these plates is 

 a displacement of the instrument between the time of the exposure on 

 the two spectra. This was guarded against by the method above de- 

 scribed. In Plates 17 and 20 there was a portion of the plate on which 

 both the spectra fell all the time, and thus gave a test of the displace- 

 ment. This was found to be zero. The other plates overlap so much 

 that there are generally two or more determinations of each line. A 

 36 



