ASTRONOMY 



scientific enquiry is to determine what proportion 

 of the total population possesses a certain amount 

 of any specified quality. 



Such enquiries belong to the domain of statis- 

 tics, and differ in no essential respect so far as 

 method is concerned from enquiries of a similar 

 nature with which we are familiar. In fact, they 

 are questions which can only be answered by 

 taking a census. But the difficulties in the way 

 of taking a census of the sky are considerable. 



So long as the only method which could be 

 employed was direct visual observation of each 

 separate star, the accumulation of a considerable 

 body of information regarding the characteristics 

 of the stars as a whole was a slow and tedious 

 process ; and it was not till the photographic plate 

 was substituted for the eye that sufficient data 

 began to be obtained to render statistical discussion 

 of the majority of the characteristics really pos- 

 sible. Not only is visual observation slow and 

 tedious, but when measurements of distance or 

 angle are involved we find most perplexing dis- 

 agreements between the results obtained by dif- 

 ferent observers, or even by the same observer at 

 different times. Moreover, when it is a question 

 of making a drawing of an extended object such 

 as a nebula, the multiplicity and delicacy of detail 

 baffle accurate delineation. 



On the other hand the photographic plate 



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