132 
PHYSICS: C. BARUS 
light this effect is slightly less marked than at maximum. It seems probable 
that we are dealing here with a shift in the maximum of intensity of the line 
spectrum of hydrogen similar to that which occurs in the continuous spectrum. 
It has been shown in the laboratory that in the case of lines belonging to the 
same series increase of temperature increases the intensity of the more refran- 
gible lines, the maximum moving toward shorter wave-lengths. In such high 
luminosity stars as the Cepheid variables such a difference as compared with 
trie sun is highly probable. 
It is of interest to note in connection with a study of the spectra of these 
variable stars that the spectra of certain stars which, with the exception of 
p Cassiopeiae, are not known to show light variation have very similar char- 
acteristics both as regards the intensity of the hydrogen and the enhanced 
lines. The most promient cases of this kind are the following: 
R. 
A. 
DEC. 
GAL. LONG. 
GAL. LAT. 
6 Canis Majoris 
7 h 
— 26° 14' 
204° 
-8° 
7 
45 
-24 37 
211 
0 
7 Cygni 
20 
19 
+39 56 
45 
+2 
Boss 5931 
22 
56 
+56 25 
75 
-5 
p Cassiopeiae 
23 
49 
+56 57 
83 
- 5 
The star 5 Canis Majoris has been found at the Lick Observatory to show a 
small variation in radial velocity. Some of these stars have spectra nearly 
identical with that of the Cepheid variables, and it is an interesting fact that 
all of them are situated very near the galactic plane and thus share in the 
peculiar distribution of the latter. 
1 Adams, W. 3., and Joy, A. H., Mi. Wilson Contr. No. 142; Astroph. J., Chicago, 46, 1917, 
(313-339). 
2 Hertzsprung, E., Astr. Nachr., Kiel, 196, 1913, (201-208). 
3 Albrecht, S., Lick Obs. Bull., Berkeley, No. 118. 
4 Duncan, J. C, Ibid., No. 151. 
5 Shapley, H., these Proceedings, 2, 1916, (208-209); also ML Wilson Contr. No. 124. 
6 Adams, W. S., and Shapley, H., these Proceedings, 2, 1916, (136-112). 
TYPES OF ACHROMATIC FRINGES 
By Carl Barus 
Department of Physics, Brown University 1 
Communicated, March 18, 1918 
The difficulty of obtaining fringes of the strictly achromatic type (i.e., two 
strong fringes with a black line between and the remaining fringes green- 
