32 
BULLETIN 1036, U. S. DEPARTMENT OE AGRICULTURE. 
the scale J. The prism AB, being split, may be opened to receive 
the liquid material between the two faces, and it is this minute 
quantity of oil or other substance which we use in measuring the 
index of refraction. In operation light falling on the mirror is re- 
flected into the prism AB. After passing through B, if no material 
is contained between the prism faces A and B, the light is totally 
reflected by the polished surface of A, and no light can pass into the 
spyglass above. If material is present, the light is refracted by this 
material, and this refracted light passes through the prism A into 
the spyglass, where it appears as a light portion above a shadow. 
The lever carrying the glass L and the split prism are then moved 
until the junction of light and shadow appears to be on the cross- 
hairs with which the spyglass is pro- 
rovided. The index of refraction is 
then read from the scale J by means 
of the reading glass L. No calcu- 
lations are necessary. 
With pure material the index of 
refraction has been used in deter- 
mining the structure and character 
of carbon compounds, but with creo- 
sote it is merely a measure of a phys- 
ical property of the oil which is easy 
of attainment. The same combi- 
nations of the same substances will 
always give the same index of re- 
fraction at a fixed temperature ; but 
it does not always follow that, 
because the same index of refraction 
is obtained, two oils are of identical 
composition. The refractometer 
finds a very large use in commercial 
testing, particularly of oils, fats, and low-melting waxes, because 
exceedingly rapid as well as accurate measurements can be made. 
The index of refraction was taken on all fractions above 235° C, 
up to the point at which the distillate was no longer liquid at 60° ( 
the temperature at which these measurements were made. T] 
values for the index of refraction are affected by the temperatu 
to a considerable extent, and the higher the temperature the low* 
the index of refraction. It is, therefore, necessary either tin 
measurements of this physical property be taken at a constant 
temperature or else that a correction be applied to change the ob- 
served reading to that at a standard temperature. The tempera- 
ture factor for changing index of refraction values varies with differ- 
ent substances; hence, correction factors for creosote would be as 
Fig. 12.— The Abbe refractometer. 
