ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
469 
A colourless liquid and a colourless solid, which have the same 
refractive indices for an intermediate colour of the spectrum, e. g. for 
the D line, show a considerable difference in the dispersion ; the value 
of n F — n G is greater for the liquid than for the solid. Consequently., 
by observation in white light, the yellow rays at grazing incidence will 
suffer no deviation and will give no indication of the boundary between 
the solid and liquid, but will form a direct image of the source of light 
in the hinder focal plane of the objective. The other rays, however, 
suffer a more or less marked deviation since slight differences in the 
refractive indices under these circumstances produce a considerable 
difference in the limiting angle of total reflection corresponding to the 
individual rays. Denoting the latter angle by e, the refractive indices 
of the liquid by n, those of the solid by n r , we have for the colours at 
C and F of the spectrum the equations : 
where n G < n’ G and n F > n f F . Since the sine of the angle near 90° 
changes only very slowly, a considerable change of the limiting angle 
corresponds to a slight difference of the refractive indices. 
The red rays at grazing incidence will thus be deviated towards the 
solid, the blue towards the liquid, as seen in fig. 83. 
Accordingly, if the Microscope be adjusted on the upper plane of the 
preparation (AB in the fig.), the edge of the glass will appear reddish 
n G 
sin e 0 = — , sin e F 
n c 
Fig. 83. 
1896 
k 2 
