772 Prof. H. Rubens and Mr. H. Hollnagel on 



Resistrahlen (figs. 3-6) is indicated in figs. 3 a, 4 a, 5 a, anrl 

 6 a, where a reverse calculation from the energy curve gives 

 us a series of pictures of the interference graphs. As is 

 evident, they are in fair agreement with the observed results. 



Results of the Wave-length Measurements. 

 Reststrahlen of Rock-salt. 



Six series of observations were made on various positions 

 of the screw of the dividing engine *. In fig. 3 the two 

 curves a and /3 refer to the series observed with the plane- 

 parallel and wedge-shaped quartz plates respectively ; in 

 the main, observations were made with the former, for they 

 were less prone to warping when fastened in place. The 

 marked similarity and regularity of the two curves is note- 

 worthy. In the two tables which follow the positions of the 

 maxima and minima for these Reststrahlen are summarized. 



Series a. 



Series /3. 



Maxima. 



Minima. 



a 



b 



c 



d 



e 



/ 



1 :::::: 



i 



k 



1 



87-0 

 82-0 

 76-9 

 71-7 

 65-7 

 60-5 

 55-6 

 510 

 46-0 

 40-0 

 34-1 



a! 



V 



c' 



W 



e' 



/ 



9' 



h! 



Id. ..... 



1' 



84-4 

 79-2 

 74-0 

 68-6 

 63-0 

 580 

 533 

 48 7 

 434 

 370 

 317 



Maxima. 



Minima. 



a 



b 



c 



d 



e 



/ 



9 



h 



i 



k 



1 



87-0 

 82-0 

 76-8 

 716 

 656 

 603 

 557 

 50-8 

 46-0 

 40-2 

 34-3 



a' 



b' 



c' 



d' 



e' 



/ 



9' 



h' 



i' 



** 



V 



84-6 

 792 

 74-0 

 68-9 

 (52-8 

 580 

 536 

 48-6 

 44-0 

 370 

 322 



To facilitate a comparison of the results, in each case the 

 point a is displaced to a position corresponding to the same 

 division of the micrometer drum, though in reality the 

 various series were obtained on entirely different portions of 

 the screw. In determining the distance between two corre- 

 sponding points, A, only the most sharply pronounced maxima 

 and minima were considered ; in the present instance, a and A, 

 b and t, a' and h\ V and i\ &c. From these, n = 14 and 

 A = 35-9. 



* A Nernst lamp served as a light-source instead of the Welsbach 

 mantle in one of these series; the two bands were present as in the 

 others, and the wave-lengths obtained were in good agreement with 

 those here calculated. 



