§ 38-] MICROSCOPIC INVESTIGATION OF CRYSTALS. 59 
(b) Lower refraction —the bright disc moves in the opposite direction 
to the mirror. 
Again, on slowly raising the objective : 
(a) Higher refraction —the bright disc gets smaller and smaller ; black 
contour appears to move inward. Bright disc last to dis¬ 
appear. 
( b) Lower refraction —bright disc gets larger and larger; black contour 
appears to move outward , and is the last to disappear. 
Precisely the same kind of phenomenon is observed in determining the 
refractive index of crystals. The crystal or crystals, if large, are crushed 
—for what is required is minute transparent bodies—and immersed 
in one or other of the liquids, or mixture of the liquids, of known re¬ 
fractive index set out in the following table. The crystal will have either 
a higher, a lower, or an equal refraction to the surrounding medium. 
The latter is only likely to be the case if the investigator knows the re- ' 
tractive index of the crystal, and selects an equally refracting liquid. 
If the crystal is found to be of a higher refraction than the liquid, then a 
still higher refracting liquid is selected, and so on until the actual refrac¬ 
tion is obtained. This is known by the want of visibility of the crystal 
and the disappearance of the contours. Besides which, in most cases 
the media used have a greater dispersive power than the crystals, and near 
the end-point coloured fringes appear. The usual conditions are, the 
liquid and solid have the same refractive index for yellow-green rays, 
but the liquid will have a higher n for blue rays than the solid, and the 
latter a higher n for red rays than the liquid. With axial light, crystals, 
red fringe inside, blue outside ; or with oblique light, bluish on one side, 
reddish on the other. On raising the objective, the red fringe moves 
inward, the blue outward. The best results are obtained by means of 
monochromatic light. 
TABLE I.—LIQUIDS FOR THE DETERMINATION OF 
THE REFRACTIVE INDICES OF SOLIDS BY THE IMMERSION METHOD 
(CHAMOT). 
Index of 
Refraction. 
Name. 
Approximate 
Boiling-point. 
Approximate 
Density. 
1-32 
Methyl alcohol. 
66 ° 
0-79 
1-36 
Ethyl ether 
35° 
0-71 
1-37 
„ alcohol . 
78° 
0-79 
1 40 
Amyl „ 
132° 
0-83 
1-44 
Chloroform 
61° 
1-48 
1-46 
Carbon tetrachloride 
76° 
1-59 
1-46 
Cajeput oil 
174° 
0-92 
1-47 
Glycerine .... 
290° 
1-61 
1-47 
Turpentine 
155° 
0-86 
