1891. | NEW YORK ACADEMY OF SCIENCES. 1 SE 
The following paper was read, entitled 
STELLAR PHOTOMETRY. 
BY HENRY M. PARKHURST. 
(Abstract. ) 
Estimations of the brightness of stars are liable to considerable 
error of judgment, causing au irregularity in the scale; and there 
is no tendency in the repetition of estimations to reduce this 
error. Any mode of measurement, however imperfect, if free 
from systematic errors, tends to make the scale of magnitudes 
uniform. The basis of photometric measures is the reduction 
of the observed star, by diminishing the aperture or by other 
means, either to invisibility or to equality of brightness with a 
standard star. The Meridian Photometer employed at Harvard 
Observatory compares the observed stars either with Polaris it- 
self or with other standard polar stars previously compared with 
Polaris. This method appears to be free from systematic error, 
but has not yet been applied to the fainter telescopic stars. 
Various methods which have been employed to reduce the effec- 
tive aperture were described, with the details necessary to secure 
accuracy. The illumination of the sky, whether by twilight, 
moonlight, or even by ordinary starlight, causes a large sys- 
tematic error, unless this is specially provided against. The 
photometer used by the speaker in asteroid observations and in 
variable star observations avoids this difficulty by making the 
illumination the same with stars of all magnitudes. ‘The loga- 
rithmic caps placed over the object-glass make the scale uni- 
form. Other details described make bias impossible, bright 
stars and faint stars appearing precisely alike just before their 
extinction. ‘This method has been employed for stars about 
two magnitudes fainter than could be measured with the Meri- 
dian Photometer. 
Another mode of extinguishing observed stars is by the 
wedge, of neutral tint glass, which is capable of measuring 
stars nearly two magnitudes fainter still. But the scale of the 
wedge is so much affected by illumination that even a change of 
magnifying power causes a considerable change of scale. ‘This 
method is especially adapted for occasional work, requiring very 
little apparatus and very little adjustment, but is purely differ- 
ential, requiring standards to be well determined by other 
means. The paper concluded with a description of the appara- 
tus by which the scale of the wedge could be determined with 
-accuracy for stars too faint to be measured by other means. 
