586 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The mean of the two readings on the sector gives, direct, the 

 refractive index of the fluid (to the third decimal place), for the 

 then temperature of the instrument and for the Fraunhofer line D. 

 By estimating the fraction of the intervals the fourth decimal may be 

 obtained. 



The mean of the drum-readings gives the value of z from which 

 the dispersion of the fluid for the colour interval between D and F 

 may be obtained from the dispersion table which accompanies the 

 instrument, and gives the value of the quantities A, B, and cr of the 

 formula at p, 584, for every reading of the sector and the drum. The 

 elements on which the figures of the table have been compnted are 

 taken in such a way, that the formula 



8 ji = A 4- B o- 



gives the dispersion of the fluid for the interval from D to F of the 

 spectrum, i. e. the difference of the refractive indices for the Fraunhofer 

 lines F and D. The value of the factor o- corresponding to z is to be 

 taken with a negative sign when z exceeds 30. 



The index of the alhidade is properly corrected if pure water at 

 about 18^ C. gives as the mean of the two readings n = 1' 3330. If 

 the index should have been displaced it can be adjusted by loosening 

 the two screws at the back of the alhidade, by which the index is 

 attached to it, and shifting the latter until the proper reading with 

 water is obtained." 



It is also pointed out that it is imperatively necessary that in 

 cleaning the prisms (with water or alcohol) only soft and very clean 

 linen should be used, and that, as the prisms are made of heavy and 

 therefore easily injured flint-glass, they should be cleaned immediately 

 after use.* 



Two simplifications of this instrument are also described by 

 Professor Abbe, enabling the refractive index only to be determined. 

 They are both intended to be held in the hand. One has the sector 

 and scale, with a direct vision prism over the eye-glass, and has the 

 full range of scale. In the other the sector is replaced by an eye- 

 piece scale, and is limited to fluids of refractive indices between 

 1 • 30 and 1 • 43. It is a very handy instrument for use with aqueous 

 or saline solutions. All the forms can be made use of for readily 

 determining whether substances have been adulterated or are pure, 

 or the degree of concentration of solutions such as sugar. 



As, however, these two forms are of more limited use, and in 

 particular do not admit of the determination of the dispersive powers, 

 it is imnecessary to give here any more detailed description. 



The paper contains very full descriptions of all the three instru- 

 ments, with directions for use, and elaborate discussions on the 

 principles and limits of exactness of the methods, and on the in- 

 fluence of the errors of observation on the results. It also con- 



* Formerly strips of paper were placed between the prisms to keep them a 

 little apart, but recently the movable prism has been ground slightly concave so 

 that the paper is not necessary. 



