232 ELEMENTARY CHEMICAL MICROSCOPY 



until the dark shadow extends approximately to the center of 

 the field, the phenomena seen will be as indicated in Fig. 139. 

 The crystal H of higher index than the liquid appears dark on 

 the dark side of the field and bright on the light side of the field; 

 but the crystal fragment L of lower index than the liquid 

 appears bright on the dark side of the field and dark on the bright 

 side of the field. This is as it should be from Fig. 138, since in 

 the image formed in the microscope the directions are reversed. 



If we now lower the condenser a reversal of all the above 

 phenomena takes place. It is therefore always wise to check 

 the results recorded with condenser raised by lowering the con- 

 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. 



The values obtained for n vary with the wave-length of the 

 light employed and the temperature at which the measurements 

 are made. In accurate work, therefore, it is essential to employ 

 monochromatic light and to correct for temperature; but in the 

 routine work of an analytical laboratory, observations made at 

 room temperatures with daylight are sufficiently exact for our 

 purposes. In order to convert monochromatic values to those 

 of different wave-length it is sufficiently exact for our purposes 

 to assume for solids that n increases by 0.001 for every 10 to 

 20 wave-lengths and that for liquids this increase is 0.002. x 



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 

 1 Wright; J. Wash. Acad. 4 (1914), 389. 5 (1915), 101. 



