. Bulletin of the Department of Agriculture. Agriculture; Agriculture. BULLETI]^ 1036, U. S. DEPARTMENT OF AGRICULTURE. the scale J. The prism AB, being spHt, may be opened to receive the hquid 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
. Bulletin of the Department of Agriculture. Agriculture; Agriculture. BULLETI]^ 1036, U. S. DEPARTMENT OF AGRICULTURE. the scale J. The prism AB, being spHt, may be opened to receive the hquid 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-meltuig 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° C, the temperature at which these measurements were made. The values for the index of refraction are affect
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