330 Mr. T. P. Dale on the Index of Refraction and 



to continue the investigation with the data at hand, and to 

 calculate the value of the limit as given by the above equa- 

 tions, using as data two observed indices only. The results 

 are given in the tables, and will be seen to include a con- 

 siderable number of fluids differing widely in specific gravity, 

 optical properties, and chemical constitution. The list it will 

 be noticed includes certain isomeric bodies and also several 

 differing by a single component. 



The first fluid examined was bisulphide of carbon at diffe- 

 rent temperatures, the data for which were furnished by Dr. 

 Gladstone to me for the purpose of this investigation. It 



will be seen that the quantity —j- is practically a constant. 



The same result is observable in the case of mint hydro- 

 carbon and benzene. The range of temperature is not in- 

 deed very large, but within the limits taken it is evident that 

 the specific gravity and the quantity v—1 increase or diminish 

 pari passu, and there is but little doubt that this relationship 

 obtains generally. It would be very desirable indeed to have 

 longer ranges of temperature * if these could be procured 

 without corresponding errors which high temperature would 

 almost certainly have a tendency to introduce. 



It is of interest evidently, in the case of a highly dispersive 

 substance, to determine how far the different indices yield the 

 same limit, as they clearly should do if the formulas are exact. 

 For the determination of this a value of was chosen which 

 should yield the value which would satisfy 6 A ; and then wave- 

 lengths of lines BCDEF and G in free aether and in the 

 medium were treated so as to find 6 B &c. If the sine and arc 

 give the same result, then calculation and observation agree. 

 It will be seen that in the case of CS 2 there is an outstanding 

 difference which amounts to not quite three minutes of arc, 

 and increases from A to H. This is equivalent to saying that 

 the formulas give correct results to the third place in the case 

 of CS 2 . This is not so good as the rock-salt in Table II. The 

 index of Gr is most affected. The difference was noticed by the 

 late Professor Baden Powell, in his ' Undulatory Theory as 

 applied to the Dispersion of Light,' London, 1841. This 

 matter deserves further investigation, but it is a matter also 

 which will involve a considerable amount of calculation; thus 

 it seemed right to postpone this for the present, as being better 



* This is especially desirable near boiling and freezing points. It is 

 possible tbat the specific gravity at boiling-point might reveal relations 

 at present masked by the arbitrary character of specific gravities taken 

 at any temperature convenient for observation. 



