of the lines of the Solar Spectrum. 183 
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 pub- 
lished and as all observers so far seem to accept the measures 
of Angstrém, 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 determination. 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 grat- 
ings 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 mount- 
ing, 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. ume micrometer has no 
errors of run or period exceeding the g5455 inch. The prob- 
’ able error of a single setting on a good clear line is about 
7 of the wave length. 1” of are is about :0012 inch. 
The D line in the second spectrum is ‘17 inch or 4:4™™ 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 ‘principal standard lines, and the chance of error is oreater 
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 readings together so as to 
include any distance, even the whole spectrum. The wave 
length is calculated for a fixed micrometer constant and then 
corrected so as to coincide everywhere very nearly with the 
standards. I suppose the probable error of the relative deter- 
minations with the weight 1 in my table to be not far from 
1 in 500,000. Angstrd6m thinks his standard lines have an 
accuracy of about 1 in 50,000 and ordinary lines much less. 
_ As to the absolute measure, it is now well determined that 
Angstrém’s figures are too small by about 1 part in 6000. 
This rests: lst, on the determination of Peirce made for the 
U.S. Coast Survey with Rutherfurd’s gratings and not yet 
completely published; 2nd, on an error made by Tresca in the 
length of the standard meter used by Angstrém* which in- 
creases his value by about 1 in 7700; 8d, on a result obtained 
* Thalen, Sur Spectre du Fer Sociéte Royale des Sciences d’Upsal, September, 
1884, p. 25. 
Am. Jour. Sct.—Tuirp SERIES, VoL. XXXIII, No. 195.—Marcg, 1887. 
12 : 
