382 ASTRONOMY 



would obviate dependence on that tool of many defects, the human 

 eye. No use seems to have been made, so far, of this method. 



The next question to be considered is, what use should be made of 

 these various measures of the light of the stars? The most obvious 

 application of them is to variable stars. While the greater portion of 

 the stars undergo no changes in light that are perceptible, several 

 hundred have been found whose light changes. A natural classifica- 

 tion seems to be that proposed by the writer in 1880. A few stars ap- 

 pear suddenly, and are called new stars, or novae. They form Class I. 

 Class II consists of stars which vary by a large amount during periods 

 of several months. They are known as variable stars of long period. 

 Class III contains stars whose variations are small and irregular. 

 Class IV contains the variable stars of short period, and Class V the 

 Algol variables, which are usually of full brightness, but at regular 

 intervals grow faint, owing to the interposition of a dark companion. 

 Twenty years ago, when photography was first applied to the discov- 

 ery of variable stars, only about two hundred and fifty of these objects 

 were known. Since then, three remarkable discoveries have been 

 made, by means of which their number has been greatly increased. 

 The first was by Mrs. Fleming, who, in studying the photographs of 

 the Henry Draper Memorial, found that the stars of the third type, 

 in which the hydrogen lines are bright, are variables of long period. 

 From this property she has discovered 128 new variables, and has also 

 shown how they may be classified from their spectra. The differ- 

 ences between the first, second, and third types of spectra are not so 

 .great as those between the spectra of different variables of long 

 period. The second discovery is that of Professor Bailey, who found 

 that certain globular clusters contain large numbers of variable stars 

 of short period. He has discovered 509 new variables, 396 of them 

 in four clusters. The third discovery, made by Professor Wolf of 

 Heidelberg, that variables occur in large nebulae, has led to his dis- 

 covery of 65 variables. By similar work, Miss Leavitt has found 295 

 new variables. The total number of variable stars discovered by 

 photography during the last fifteen years is probably five times the 

 entire number found visually up to the present time. Hundreds of 

 thousands of photometric measures will be required to determine 

 the light-curves, periods, and laws regulating the changes these 

 objects undergo. 



A far more comprehensive problem, and perhaps the greatest in 

 astronomy, is that of the distribution of the stars, and the constitu- 

 tion of the stellar universe. No one can look at the heavens, and see 

 such clusters as the Pleiades, Hyades, and Coma Berenices, without 

 being convinced that the distribution is not due to chance. This view 

 is strengthened by the clusters and doubles seen in even a small tele- 

 scope. We also see at once that the stars must be of different sizes, 



