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 0.002.1 
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 
^ Wright; J. Wash. Acad. 4 (1914), 389. 6 (1915), loi. 
