238 
ASTRONOMY: F. H. SEA RES 
this will be true if stars of intermediate or late type are present. The results 
afforded by observations of color would however be useful as a control on spec- 
troscopic or trigonometric determinations of parallax, although they might not 
add greatly to our knowledge of stellar distances. But if the method depend- 
ing on color measurements could be extended in such a way as to obviate the 
necessity of direct spectral observations, the advance would be of importance, 
for it could then be applied to faint stars beyond the reach of both trigono- 
metric and spectroscopic methods of measuring distance. 
Theoretically such a procedure seems not impossible. Suppose, for example, 
that we observe three spectral regions instead of two, and that we combine the 
results in such a way as to yield two exposure-ratios, log R (Xi, X2) and 
log R (Xi, X3), Xi, X2, and X3 representing the effective wave-lengths of the three 
regions in question. Presumably both these quantities would be functions 
of the spectral type 6* and the absolute magnitude M; hence, we could write 
logi^(Xi,X2) = Fi {S, M) 
\ogR{\,M = ¥2{S,M) 
from which it might be possible to determine both S and M , and therefore, 
finally, the parallax, with the aid of the usual formula 
SlogTT = M — m — S 
in which m is the apparent magnitude. 
It must be admitted, however, that the trials thus far made hold out no 
great promise of success, owing to the fact that the coefficients in the two equa- 
tions involving 5 and M are so nearly proportional as to render the solution 
indeterminate. The spectral regions used were in the violet, blue, and yel- 
low. These were tried first because of photographic considerations; but others 
may prove better adapted to the requirements of the problem, and it is pro- 
posed to give the question further attention. 
1 Adams, W. S., Mt. Wilson Contr. No. 78, Astrophys. /., Chicago, 39, 1914 (89-92). 
Also results by van Rhijn; quoted by Kapteyn in Mt. Wilson Contr. No. 83; Astrophys. J., 
40, 1914 (187-204); and later given in detail in "Derivation of the Change of Color with Dis- 
tance and Apparent Magnitude," Groningen, 1915. 
Adams, W. S., and Arnold Kohlschutter, Mt. Wilson Contr. No. 89, Astrophys. J. Chicago, 
40, 1914 (385-398). 
Monk, G. S., Mt. Wilson Contr. No. 119, Astrophys. /., 44, 1916 (45-50). 
Seares, F. H., Pub. A. S. P., San Francisco, Calif., 30, 1918 (99-133). 
Lindblad, Bertil, Arkiv for Mathematik, Astronomi och Fysik, Stockholm, 13, No. 26, 1918 
(1-75). 
2 Seares, F. H., loc. cit. 
3 Seares, F. H., These Proceedings, 2, 1916 (521-525). 
4 Adams, W. S., and A. H. Joy, Mt. Wilson Contr. No. 142, Astrophys. J., 46, 1917 (313- 
339). 
5 Kapteyn, J. C, Mt. Wilson Contr. No. 147, p. 76, Astrophys. J., 47, 1918 (10^133, 146- 
178, 255-282), p. 266. 
6 Kapteyn, J. C, Mt. Wilson Contr. Nos. 82, 147, Astrophys. /., 40, 1914 (43-126), 46, 
1918 (104-133, 146-178, 255-282). 
