PENDING PROBLEMS OF ASTRONOMY. 63 



utilize the rays, or unknown worlds of which we have no cognizance, 

 beyond the stars. 



Now, for my own part, I am very little troubled by accusations of 

 wastefulness against Nature, or by demands for theories which will 

 show what the human mind can recognize as " use " for all energy ex- 

 pended. "Where I can perceive such use, I recognize it with reverence 

 and gratitude, I hope ; but the failure to recognize it in other cases 

 creates in my mind no presum^^tion against the wisdom of Nature, or 

 against the correctness of an hypothesis otherwise satisfactory. It 

 merely suggests human limitations and ignorance. How can one with- 

 out sight understand what a telescope is good for ? 



At the same time perhaps we assume with a little too much confi- 

 dence that, in free space, radiation does take place equally in all di- 

 rections. Of course, if the received views as to the nature and conduct 

 of the hypothetical " ether " are correct, there is no possibility of 

 questioning the assumption ; but, as Sir John Herschel and others 

 have pointed out, the properties which must be ascribed to this 

 "ether," to fit it for its various functions, are so surprising and almost 

 inconceivable that one may be pardoned for some reserve in accepting 

 it as a finality. At any rate, as a fact, the question is continually 

 started (the idea has been brought out repeatedly, in some cases by 

 men of recognized scientific and philosophic attainment), whether the 

 constitution of things may not be such that radiation and transfer of 

 energy can take place only between ponderable masses ; and that, too, 

 without the expenditure of energy upon the transmitting agent (if 

 such exist) along the line of transmission, even in transitu. If this 

 were the case, then the sun would send out its energy only to planets 

 and meteors and sister-stars, wasting none in empty space ; and so its 

 loss of heat would be enormously diminished, and the time-scale of 

 the life of the planetary system would be correspondingly extended. 

 So far as I know, no one has ever yet been able to indicate any kind 

 of medium or mechanism by which vibrations, such as we know to 

 constitute the radiant energy of light and heat, can be transmitted at 

 all from sun to planet under such restrictions ; and it is very difficult 

 to see how any such limited transmission, confined to the lines of 

 gravitational force, could be reconciled with the law of inverse squares. 

 That this law of radiation actually holds in interplanetary space is of 

 course demonstrated by the fact that the calculated brightness of a 

 planet at different places in its orbit and varying distances from the 

 earth agrees with the results of photometric observation. Still, one 

 ought not to be too positive in assertions as to the real condition and 

 occupancy of so-called vacant space. The " ether " is a good working 

 hypothesis, but hardly more as yet. 



I need not add that a most interesting and as yet inaccessible 

 problem, connected with the preceding, is that of the mechanism of 

 gravitation, and, indeed, of all forces that seem to act at a distance '. 



