39 



ON" THE RELATIVE WAVE-LENGTH OF THE LINES OF THE 



SOLAE SPECTRUM 



[American Journal of Science [3J, XXXIII, 182-190, 1887 ; Philosophical Magazine 

 [5], XXIII, 257-265, 1887] 



For several years past I have been engaged in making a photographic 

 map of the solar spectrum to replace the ordinary engraved maps and 

 I have now finished the map from the extreme ultra violet, wave-length 

 3200, down to wave-length 5790. In order to place the scale correctly 

 on this map, I have found it necessary to measure the relative wave- 

 lengths of the spectrum and to reduce it to absolute wave-lengths by 

 some more modern determination. I have not yet entirely finished the 

 work, but as my map of the spectrum is now being published and as 



O 



all observers so far seem to accept the measures of Angstrom, I have 

 decided that a table of my results would be of value. For as they stand 

 now they have at least ten times the accuracy of any other determina- 

 tion. This great accuracy arises from the use of the concave grating 

 which reduces the problem of relative wave-lengths to the measure of 

 the coincidences of the lines in the different spectra by a micrometer. 



The instrument which I have employed has concave gratings 5 or 6 in. 

 diameter, having either 7200 or 14,400 lines to the inch and a radius of 

 21 ft. 6 in. By my method of mounting, the spectrum is normal where 

 measured, and thus it is possible to use a micrometer with a range of 

 5 inches. The spectrum keeps in focus everywhere and the constant 

 of the micrometer remains unchanged except for slight variations due 

 to imperfections in the workmanship. The micrometer has no errors 

 of run or period exceeding the -J^TTF inch. The probable error of a 

 single setting on a good clear line is about ^nrVur ^ the wave-length. 

 1" of arc is about -0012 inch. The D line in the second spectrum is -17 

 inch or 4-4 mm. wide. Determinations of relative wave-length of good 

 lines seldom differ 1 in 500,000 from each other and never exceed 1 in 

 100,000, even with different gratings. This is, of course, for the prin- 

 cipal standard lines, and the chance of error is greater at the extremities 

 of the spectrum. The interpolation of lines was made by running the 

 micrometer over the whole spectrum, 5 inches at a time, and adding the 



