294 Mr. A. A. Michelson on the Application of 



If A be the difference of path at which the visibility is 

 reduced to half its value at X = 0, then 



or approximately, 



7T V 7T ' V ' 



A- V 



If we take for hydrogen v = 2000 metres per second, then 



^=22500. 



Again, if we ignore the difference in the temperature 

 (about which there is considerable uncertainty), at which the 

 other substances were examined, the velocities v would vary 

 inversely as the square root of the atomic weight, and the 

 number of waves in the difference of path at which the 

 visibility is 0'5 is therefore 22500 »Jm. 



Considering the difficulties and uncertainties of the problem, 

 the following Table shows a remarkable agreement between 

 the values actually found and the calculated results *. 



Substance. 



At. Wt, 



A. 



A. 



»=!■ 



N. (Calc). 



H r 



1 

 1 



16 



23 



23 



23 



23 



23 



65-5 



65-5 

 1120 

 112-0 

 112-0 

 200-0 

 200-0 

 200-0 

 2000 

 203-6 



656 

 486 

 616 

 616 

 589 

 567 

 515 

 498 

 636 

 481 

 644 

 509 

 480 

 579 

 577 

 546 

 436 

 535 



190 



8-5 



34-0 



66-0 



800 



620 



440 



55-0 



660 



47 



1380 



1200 



64-0 



2300 



]54-0 



2300 



100-0 



220-0 



30000 



18000 



55000 



107000 



133000 



109000 



85000 



110000 



104000 



98000 



215000 



236000 



134000 



400000 



270000 



420000 



230000 



400000 



22500 

 22500 

 80000 

 108000 

 108000 

 108000 

 108000 

 108000 

 382000 

 182000 

 238000 

 238000 

 238000 

 317000 

 317000 

 317000 

 317000 

 322000 



H b 



O 



Na r 



Na y 



Na OT 



Na K ' 



Na g " 



Zn r 



Zn b 



Cd r 



Cd g 



Cd b 



H> ' 



Hg v " 



Hg„ 



He* 



Tl 





* It should be stated that the value of A for the yellow sodium-line, 

 if taken from the curve, would be much larger than that given. The 

 latter was the mean of a number of observations taken within the past 

 month. As has been stated before, this particular curve has not been 

 obtained since last year. A few other substances, very difficult to 

 examine, either because the lines are too feeble, or because the spectrum 

 is so unstable, have given results not quite so consistent as the above, 

 though all are of the same order of magnitude as that required by theory. 



