156 CARNEGIE IKSTITUTION 



estimate of equality b}' the eye. Many of the errors thus introduced 

 may be ehrainated by substituting for the eye a radiometer, bolome- 

 ter, or thermopile and measuring the proportion of heat intercepted 

 by an image on a photographic film. Another piece of work which 

 is sadly in need of revision in accordance with modern methods is 

 the precise determination of the relative brightness of the Sun and 

 other members of the solar system as compared with the stars. 



Variable Stars. — One of the most important applications of pho- 

 tometrj' is to the study of stars which vary in light. Variable stars 

 may conveniently be divided into new stars, variable stars of long 

 period, those having small and irregular variations, those of short 

 period, and the Algol stars. 



There is no obvious way of aiding the prompt discovery of new 

 stars, and when such an object is found the general interest in it 

 insures an abundance of observations. Except in the case of stars 

 bright enough to be visible to the naked e3'e, all recent new stars 

 have been found by the aid of photography, and nearly all of our 

 knowledge of them has been found by photographic processes. 



The study of variable stars of long period is one that can be 

 greatly aided by judicious appropriations. The laws regulating the 

 changes in these bodies are unknown and appear to be complex. 

 They can be determined only from long series of observations ex- 

 tending over many years. If not made now we may be blamed by 

 future astronomers who may justly say, "How much we could do 

 now if our predecessors had not neglected their opportunities." 

 Bach star should be observed at least once a month. The observa- 

 tions are easily made, require but little apparatus in addition to a 

 telescope, and much valuable work has been done on them by ama- 

 teurs. Many of the large telescopes of the country are now idle 

 during a great portion of the night, for want of the means of em- 

 ploying a competent observer. A small sum would pay the salary 

 of an advanced student who might thus at the same time continue 

 his studies and do useful work. Careful superintendence and coop- 

 eration would be required to insure a proper distribution of the ob- 

 servation of each star, otherwise as at present some stars would be 

 neglected, others observed more frequently than is necessary. The 

 same comparison stars, the same system of photometric magnitudes, 

 and the same form of publication should be used by all the observers. 



The observations of the variable stars of short period stand on a 

 wholl}^ different basis. These stars appear to vary with perfect uni- 

 formity, except for slight variations in the length of the period. It 



