Appreciation of Ultra-visible Quantities. 42$ 



The nickel line which is nearly midway between the two 

 D lines ; and the line at \=5892'6. This is the 

 second from the nickel line towards D 2 , of the eleven 

 lines which are at all times visible between the two 

 D lines. 



The least refrangible constituent of the triple line at 

 X = 5328'7. 



This last is about the closest double that my spectroscope 

 will resolve. There is no micrometer on my instrument, so 

 that I cannot give measures, but I estimate the coarsest of 

 these, those first mentioned, to have a difference of wave- 

 length under two eleventhet-metres — and in the closest, that 

 last mentioned, it cannot be more than a very few twelfthet- 

 nietres. Most of them could be measured with a good 

 micrometer. 



This can be accomplished with one of the smaller of 

 Professor Rowland's splendid gratings ; and he himself and 

 other observers have carried matters farther, by taking pho- 

 tographs with the best of his great six-inch concave gratings. 

 This may give some idea of the marvellous precision of this, 

 the latest and most searching appliance for exploring Nature. 

 By it a brilliant series of discoveries have already been made 

 in stellar astronomy ; and we may anticipate still greater 

 achievements from the distance to which it can throw its 

 plumb-line into the obscure depths of molecular events. 



X. Time Relations. 



The fragments of time that can be appreciated with accuracy 

 in this way are even more wonderful in their minuteness than 

 are the differences of length. Time relations, however, lie 

 somewhat outside the scope of the present essay ; but they, 

 too, should be carefully pondered by anyone who wants to 

 know what Nature really is. And after thus taking the best 

 survey that he can, he should bear in mind that all he can do 

 is to gauge the little that man has been fortunate enough to 

 detect ; and that far more may lie beyond the ken of any human 

 being than the immense range which now lies within it. He 

 should also reflect that the few molecular events that are 

 already known succeed one another with such astonishing- 

 rapidity that the swiftest visible motions are, in relation to 

 them, as sluggish and as gradual in their progress as are the 

 changes in the configurations of the constellations owing to 

 the proper motions of the fixed stars, in their relation to us 

 and to the events we can see occurring about us on the earth. 

 In fact, the thousandth of one second of time is, in relation 



2G2 



