1891.] NEW YORK ACADEMY OF SCIEXGES. 117 



The following paper was read, entitled 



STELLAR PHOTOilETRY. 

 BY HENRY M. PARKHURST. 



(Abstract.) 



Estimations of the brightness of stars are liable to considerable 

 error of judgment, causing an irregularity in the scale; and there 

 is no tendency in the repetition of estimations to reduce this 

 error. Any mode of measurement, liowever 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 t)ie 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. 



