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 affected by the temperature 

 to a considerable extent, and the higher the temperature the lower 

 the index of refraction. It is, therefore, necessary either that 

 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. 



