190 ELEMENTARY CHEMICAL MICROSCOPY 



denser; moreover the phenomena are much more distinct with 

 lowered condenser. 



There is little chance for an error of judgment if the student 

 will start with condenser raised and stopped down, and first 

 slowly raise the objective, noting the direction of apparent move- 

 ment of the contour bands or halo. Next test with oblique light 

 and note the relative position of the dark contours with respect 

 to the dark half of the field and finally lower the condenser and 

 test again with oblique light. All three of the sets of observa- 

 tions should be in accord. The student should also learn to use 

 a finger below the condenser to obtain oblique illumination and 

 thus save time. 



Since most of the liquids employed for the determination of 

 refractive index by the immersion method have a greater dis- 

 persive power than the solids, at the end point in the immersion 

 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 diagramed in 

 S, Fig. 108, 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 



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



