DEVELOPMENTS IN ASTRONOMY—PLASKETT, _ »2B8 
of star clusters and nebule ever made have already been obtained 
with the instrument, and its light efficiency in spectrographic work 
is wonderful. It can obtain in five minutes a spectrum of a fifth 
magnitude star that requires with our refractor over an hour. It is 
no wonder that such an instrument excited the envy of all astrono- 
mers who saw it, and Prof. Ritchey was pardonably proud of his 
masterpiece. 
We turn from this to, comparatively speaking, a rather insignificant 
instrument, for measuring the brightness of the stars. The subject 
of stellar photometry has always been a difficult one, as all the 
photometers hitherto devised have depended upon eye estimates 
or comparisons of the relative brightness of the star with either 
another star or an artificial light, made by ingenious devices to 
resemble and be brought close beside the star to be measured. 
There is, in ail such methods, the possibility of psychological errors, 
and it has not been possible to obtain, except in spe¢ial cases, results 
with a lower probable error than about one-tenth of a magnitude. 
In the case of the comparison of two stars brought into the one field 
and equalized in intensity by polarizing apparatus, the probable 
error is, perhaps, as low as three or four hundredths of a magnitude. 
In another method also, in which out-of-focus images of the stars are 
photographed, the density of the resulting disks have then to be 
measured by a photometer and we have errors of the same order. 
The new method, however, does not depend on eye estimates but on 
the change in electrical resistance of the element selenium when 
exposed to light. If a selenium cell is placed on the end of a telescope 
and an image of a star to be measured thrown on it, the change of 
resistance can be measured by a Wheatstone bridge arrangement and 
very accurate values of the brightness obtained. Prof. Stebbins, 
who has been working with much ability and energy on this problem 
for the iast three years, deserves mich credit for his success in a 
difficult research. He has recently made new measures of the light 
curve of the well-known variable star Algol, and the probable error 
of a determination at maximum is + .006 mag., at minimum + .023 
mag. The accuracy of his observations enabled him to detect a 
secondary minimum which had never before been seen and which 
‘indicates that the companion whose eclipse of the bright star causes 
the variability is not dark but light. Taking the most probable 
value of the parallax or distance of the star, he finds that the bright 
star, which has about the same diameter as the sun, gives 240 times 
as much light, while the faint hemisphere of the companion gives 16 
and the bright hemisphere 28 times the light of the sun. 
Such results as these are most interesting, and it is only by combi- 
nation of several different methods, in this case of the light variation 
by a photometer, the orbital elements by the spectroscope, and the 
