184 H, A. Rowland—Relative Wave-lengths 
in my laboratory with two of my gratings by Mr. Bell, which 
is published with this paper. Mr. C. S. Peirce has kindly 
placed his grating at our disposal and we have detected an 
error of ruling which affects his result and makes it nearly coin- 
cide with our own. The wave length of the mean of the two 
K lines is 
Amostromy (atlas)eoe 95 00 eee ie 5269°12 - ‘05 
cs (Corrected by Thalen) ------ 5269°80* 
Pein Ce). Be eee Se ass Sst he Ave peers 5270°16 © 
«(Corrected by Rowland and Bell). 5270-00+ 
Bell saiaige Stes ety rare eye Ae Es eee 5270-04 
These results are for air at ordinary pressures and tempera- 
tures. The last is reduced to 20°C. and 760™™ pressure. To 
reduce to a vacuum, multiply by the following: 
Fraunhofer line A C E ~ G H ; 
Correct’n factor. 1:000291 1:000292 1:000294 1°000297 1:000298 
The relation between my wave lengths and those of Ang- 
strom are given by the following, Angstrém’s value being 
from p. 31 of his memoir. 
A (edge) B (edge) C < 
Anostromimes 222 7597.5 6867:10 6717°16 656210 6264°31 
Rowland __-.:... 75938°97° 6867°38 6717°83 6562°96 6265:27 
Difference ------- — 3:5 28 67 "86 96 
Dy D, Peirce’s line 
Angstrom ea Sens 5895°13 5889°12 5708°45 5623°36 5454°84 
Row land. os 5896°08 5890°12 5709°56 5624°70 5455°68 
Difference 2422724 "95 1:00 Neat ‘1°34 "84 
E E by EF 
(Angstroms 22 2 526959 526867 518310 513878 4860-74 
Rowland ..-...-- 5270°48 5269°65 5183°78 5139:47 ‘4861-43 
Difference ------- 84 "98 63 69 69 
G 
Angstrom __----- 4702°44 4307-25 
Rowland 2) ese 4703-11 43807:96 
Difference ------- 67 fal 
The greatest variation in these differences is evidently due 
to the poor definition of Angstrém’s grating by which the 
numbers refer to groups of lines rather than to single ones. 
Selecting the best figures, we find that Angstrém’s wave 
* Thalen, Sur Spectre du Fer, Sociéte Royale des Sciences d’Upsal, September, 
1884, p. 25. + From one grating only. 
