﻿9U Prof. 0. Barus on the 



of AA. If Xi and x 2 measured on S be the positions of C for 

 identical lines of the spectrum, 



A,/D = (^ 2 -^)/2R, 



where D is the grating space and R the length ap. 



The lens I/" on R is the objective of a telescope which 

 f ocusses the coloured image of the slit at the slide 0, so that 

 it may there be viewed by an ocular preliminarily. The 

 latter is held in a standard placed on the table t' revolving 

 on a common axis p with this end of R. When the ocular 

 is replaced by the total reflectometer T, of which p is the 

 plate of glass (properly centred) to be examined and q the 

 telescope, the weak lens L'" is superfluous ; but as a rule it 

 need not be removed, and is rather advantageous in condensing 

 light when sunlight is not used. 



When sunlight is used, the lens L'" may be moved slightly 

 backward, so as to put the coloured images of the slit 

 slightly in front of a cylindrical glass vessel T of the total 

 reflectometer. If the adjustment is properly made the lines 

 are adequately visible in the telescope q, and the index may 

 be found for a given Fraunhofer line without reference to 

 the scale ss. Without sunlight, the scale ss is essential. 

 Naturally the images seen in the telescope are not sharp, 

 inasmuch as the cylindrical mantle of the bottle acts as one 

 of the lenses ; but they are clear enough for the purpose. It 

 is usually more satisfactory, however, to ignore the lines 

 and to determine the index of refraction for successive 

 positions x of the carriage C on the rail AA, given by the 

 scale ss. 



With the total reflectometer revolvable, as a whole, on its 

 axis, the two symmetrical positions of q relative to R give the 

 double angle of total reflexion, or 2<£, for the given liquid 

 (usually carbon disulphide) in which the glass plate p is 

 submerged. The graduated circle of the instrument with 

 which the telescope q is rigidly connected is available for the 

 measurement of angles. 



3. Methods. — When the air plate is used for the measure- 

 ments of the indices of refraction of the liquid, the telescope 

 q is directed along the rail R and 2(j) t found by rotating the 

 plate, p, until the two limits or ends of the visible part of 

 the spectrum successively coincide with the cross-hairs. The 

 spectrum is usually nearly homogeneous in colour; but the 

 ends are liable to show interference fringes. Simultaneously 

 the position x of the slide C and the temperature t of the 



