362 ASTRONOMY 



errors that the small quantity actually sought will emerge in the final 

 average. This will be a question of mathematical probability; and it 

 is not always easy to determine what the exact measure of this prob- 

 ability is. Yet, if we could not measure the parallaxes of the stars, 

 astronomy would lack one very important link in the chain of evidence 

 by which the reality of the earth's motion is firmly demonstrated. 

 Furthermore, if the parallaxes of the stars, and consequently their 

 distances, could not be measured with a fair approximation to the real 

 quantities, then a most important element in the foundation of stellar 

 astronomy would be wanting. Consequently astronomers are obliged 

 to measure these quantities as well as they can, and they must push 

 their methods and efforts to extremity. 



In short, astronomers are continually obliged to measure and reason 

 about quantities which cannot b^e distinctly perceived in the telescope. 

 Accordingly, from the necessities of the case, there has arisen in 

 modern astronomy what is almost a distinct science that of meas- 

 urement which absorbs a very important part of the total energy 

 expended in astronomical research. This is carried so far, in sohie 

 instances, that the astronomer sometimes seems, in a measure, to 

 lose sight of the natural phenomenon to be observed and to be quite 

 wholly absorbed in the means by which he observes it. 



The nature and necessities of modern scientific research have 

 brought about what, for the want of a better term, might be called 

 the astronomy of the unseen. The companion of Algol has never been 

 seen and never can be seen, yet it is known to exist and even its 

 dimensions and mass are known with a fair degree of probability. So 

 far as this idea concerns apparent motion on the face of the sky, its 

 means of perception are found in multiplication of measurements; 

 in variation of the methods and circumstances of those measurements; 

 and in interpretation of results. One phase of these processes is well 

 illustrated in researches upon the solar parallax. For its determina- 

 tion there are at least four distinct types of investigation, each involv- 

 ing the application of more than one method. The discordance in 

 angle between the results from each of these four sources of deter- 

 mination is too small to be perceived by the aid of the most powerful 

 telescope. In many other instances, as in the variation of the earth's 

 axis of rotation from its axis of figure, astronomy successfully reasons 

 about quantities which are actually too small to be perceived im- 

 mediately through any aids of the senses that are available. The 

 logic of this achievement rests upon the validity of the practical 

 application of the mathematical theory of probabilities to the treat- 

 ment of residual phenomena. The outcome is that the objective 

 reality of things which we cannot see may be as firmly established 

 as that of things which we can see. The entire process is essentially 



