52 LIGHT SCIENCE FOR LEISURE HOURS. 



nor unreasonable, since we find implanted in our very 

 nature and not merely in the nature of scientific 

 men a quality which causes us to take interest in a 

 variety of matters that do not in the least concern our 

 personal interests. Nor is this quality, rightly con- 

 sidered, one of the least noble characteristics of the 

 human race. 



That the determination of the sun's distance is im- 

 portant, in an astronomical sense, will be seen at once 

 when it is remembered that the ideas we form of the 

 dimensions of the solar system are wholly dependent on 

 our estimate of the sun's distance. Nor can we gauge 

 the celestial depths with any feeling of assurance, unless 

 we know the true length of that which is our sole 

 measuring-rod. It is, in fact, our basis of measurement 

 for the whole visible universe. In some respects, even 

 if we knew the sun's distance exactly, it would still be 

 an unsatisfactory gauge for the stellar depths. But that 

 is the misfortune, not the fault, of the astronomer, who 

 must be content to use the measuring-rod which nature 

 gives him. All he can do is to find out as nearly as 

 possible its true length. 



When we come to consider how the astronomer is 

 to determine this very element the sun's distance we 

 find that he is hampered with a difficulty of precisely 

 the same character. 



The sun being an inaccessible object, the astrono- 

 mer can apply no other methods to determine its dis- 

 tance directly than those which a surveyor would use 

 in determining the distance of an inaccessible castle, or 



