€2 



Lord Rayleigh on the remarkable Case of 



that 609 is passing off in the third spectrum on the same 

 side as that on which the light is incident, and 517 in the 

 third spectrum upon the other side. But the agreement of 

 3' 10 and 3*15 with the integer 3*00 seemed hardly good 

 enough, and so the matter was put aside until recently, when 

 my attention was recalled to it in reading an article by 

 Prof. Ames * on Rowland's ruling-machines, from which it 

 appeared that gratings have been ruled with three different 

 spaces, viz. 14438, 15020, and 20000 lines to the inch. If 

 we permit ourselves to suppose that the number of lines in 

 the special grating is really 15020 to the inch in place of 

 14438, the alteration would be in the right direction, 3*10 

 becoming 2*98 and 3'15 becoming 3'03, so that the mean 

 would be about correct. 



In view of this improved agreement it seems worth while 

 to consider how far the position of the bands recorded in the 

 other diagrams would accord with the formula 



X=£e(l + sm0), (2) 



taking e to correspond with ruling at the rate of 15020 to the 

 inch. In one respect there is a conspicuous agreement with 

 Prof. Wood's observations. For if X 1? A 2 are the two values 

 ■of X in (2), we have at once 



Xi + X 2 = §e, (3) 



so that the two bands move equally in opposite directions as 

 6 changes. 



The results calculated from (2) for comparison with dia- 

 grams (2) . . . . (10) (fig. 1) are given below. 



9. 



Calculation. 



Observation. 



No. 



K 



X 2 . 



X r 



,X 2 . 



2° 37' 



0°15' 



0° 5' 



0° 0' 



0° 5' 



1°15'...... 



1°53' 



2° 38' 



5° 45' 



590 

 566 

 • 564 

 564 

 561 

 576 

 582 

 590 

 620 



538 

 561 

 563 

 564 

 563 

 551 

 515 

 538 

 507 



589 

 566 

 562 

 561 



575 

 581 

 589 

 619 



537 

 559 

 561 

 561 



549 

 542 



538 

 506 



2 

 3 

 4 

 5 

 6 

 7 

 8 

 9 

 10 



* Johns Hopkins University Circular. Notes from the Physical 

 Laboratory, Ap. 1906. 



