112 Prof. Wood on the Dispersloa of Toluine. 



obtained with prisms o£ two different angles. It is at once 

 apparent that the absorption-band at wave-length '00027 

 modifies the curve to no small degree. In the original nega- 

 tives, a very slight trace of oppositely curved branches such 

 as are always present at the edges of strong absorption-bands 

 is discernible. The band is quite narrow and fairly strong, 

 yet it does not seem to modify the curve to any great degree 

 on the red side, at least the curve has nothing like the 

 steepness which it has in the remote ultra-violet. On the 

 other side of the band, however, the curve is seen to be 

 depressed to a considerable distance. The influence of the 

 band seems therefore to be unsymmetrical. This is to be 

 ascribed to the fact, I believe, that the absorption on the 

 ultra-violet side of this band is quite strong, as will be seen 

 from fig. 11. 



The plates were measured in the same manner as the nitroso 

 plates, and a table of the refractive indices is given below. 



X. 



n. 



>. 



n. 



'l'^^^ 



1-885 



2681 



1-640 



2288 



1-8503 



2750 



1-628 



2314 



1-821 



2980 



1-595 



2'6'2'2 



1-808 



3250 



1-570 



2330 



1-807 



3400 



1-554 



2372 



1-767 



3659 



1-542 



2470 



1-709 



3995 



1-526 



2527 



1-679 



4799 



1-507 



2570 



1-649 







The curve is shown graphically on Plate VI. 



Applying the dispersion formula to the results, we find, 

 neglecting the band at -00027 for the centre of the band of 

 metallic absorption in the nltra-violet, the value -000182, 

 which is not inconsistent with the photographic records. For 

 the other constants we find m = l'?>l and m^ = -77. 



The sum of these two constants should give us the dielectric 

 constant of the substance if no other bands of absorption are 

 present. 



The sum 7n + m^ = 2'14o. while the dielectric constant is 2*36 

 (determination by Palaz), from which it seems probable that 

 there is an absorption-baud in the infra-red. 



The formula expresses the dispersion very fairly except for 

 a narrow^ range immediately on the more refrangible side of 

 the band at wave-length -00027. Adding another term to 

 the formula will not help matters much in this region, in my 

 opinion, owing to the indefinite nature of the absorption. 



The substance seems to be worthy of a more complete 

 investigation. 



