280 PROCEEDINGS OP THE AMERICAN ACADEMY 



scale proper corrections for the effect of haze, moonlight, etc. He 

 may also observe a large number of known stars, and afterwards 

 reduce his scale for the evening from a discussion of their light. 



In the second method, which is tliat of Argelander, the observer 

 selects a comparison star of very nearly the same light as the star to be 

 measured, and estimates the difference in grades, a grade being a small 

 interval nearly equivalent to a tenth of a magnitude. A discussion of all 

 the observations serves to determine the intervals in grades between 

 the comparison stars. The value of one grade is then determined from 

 photometric measures of the comparison stars. According to tlie third 

 method, the observer selects two comparison stars, one a little brigliter, 

 the other a little fainter, than the star to be observed, and estimates its 

 difference in magnitudes from the brighter component, with the differ- 

 ence between the two compai'ison stars. The first of these methods is 

 the most rapid, and is well adapted to zone observations, or to any work 

 with a meridian instrument. More skill is, however, required on the 

 part of the observer than by either of the other methods. Besides being 

 able to judge of small intervals of brightness, as in the other methods, 

 he must be able to prevent any changes from taking place in his scale, 

 at least during a single evening. The second method requires less 

 skill, since the observer must merely keep the values of iiis grades con- 

 stant ; but in tlie third method even this is not needed. It is, there- 

 fore, probably the most exact, when tlie results are to be reduced by 

 photometric measures of the comparison stai's. The three methods are 

 directly comparable with those which may be used in estimating linear 

 measures. We may estimate the length of a bar directly in inches, or 

 its excess in inches over a similar bar of known length ; or, thirdly, we 

 may compare it with two bars, one a little longer, the other a little 

 shorter, and estimate its relative length compared with them. It can 

 hardly be doubted that the last of these methods would give the most 

 accurate results. When applied to the stars, the third method has 

 also an advantage in reducing the accidental errors of the photometric 

 measures, since the comparison is made with two stars instead of one. 



The light curve of a variable may then be determined as follows : — 

 Select as compai-ison stars all those of neai-ly the brightness of the 

 variable, and not too far distant, excepting any which may be thought 

 to be variable, to differ from the variable in color, or which are near 

 other stars. Photometric measures should be obtained, during the 

 period over which the observations of the variable extend, of all of 

 those stars which are used. Each star should be measured in turn 

 under precisely the same conditions, by a Zollner i)hotometer or other 



