THE DETERMINATION OF REFRACTIVE INDEX 233 



method the images usually appear surrounded by colored fringes. 

 The conditions which usually obtain are that when the liquid and 

 solid have the same refractive index for yellow-green rays, the 

 liquid will have a higher n for blue rays than the solid but the 

 solid will have a higher n for red rays than the liquid. It follows 

 that the emerging red rays will be convergent as diagrammed in 

 S, Fig. 138. while the emerging blue rays will be divergent. 1 No 

 dark contour bands will be sufficiently prominent to be noticeable, 

 but the image will exhibit a bluish fringe on the outside and a 

 reddish fringe on the inside, or with oblique light bluish on one 

 side, reddish on the other. Raising the objective will cause the 

 red fringe to move inward and the blue fringe outward. It is 

 evident that this color dispersion phenomenon enables us to still 

 further assure ourselves when we have found the liquid having 

 the same n as that of the solid under examination. 



When in the course of the experiments a marked color fringe 

 is seen with the absence of black bands, the point has been 

 reached in which liquid and solid have the same refractive index 

 for light rays of medium wave-length. To obtain more accurate 

 results recourse must be had to monochromatic light. 



In preparing a series of liquids of regularly differing refractive 

 indices for use in this immersion method, it is advantageous to 

 select those having a slightly greater color dispersion than will 

 be found in the solids to be tested. But highly dispersive liquids 

 must be avoided since the color bands or halos are then so marked 

 as to seriously interfere with the recognition of dark contours. 



Having ascertained as described above whether the crystal 

 fragment has a higher or a lower index than that of the liquid 

 first tried, and thus in which direction to proceed, a second liquid 

 whose index is probably very much nearer that of the solid is 

 chosen. The first liquid is carefully removed by absorbing it 

 with a bit of filter paper, a drop of the liquid next to be applied 

 is added and allowed to flow completely around the crystal; after 

 standing a few moments this is removed as before and a new por- 

 tion added. The preparation is tested by raising the objective 

 and by the half-shadow method to learn whether the solid or the 



1 Wright, Amer. J. Sci., loc. cit. 



