For details of the construction of these apparatus, with a description of the optical principles on which they are based, the papers above cited may be consulted. In this laboratory the instruments which have been employed are three in number, _viz._, Abbe’s small refractometer, Pulfrich’s refractometer using yellow light, and the oleorefractometer of Amagat-Jean. A brief description of the methods of manipulating these instruments is all that can be attempted in this manual. =295. Refractive Index.=—Refractive index is an expression employed to characterize the measurement of the degree of deflection caused in a ray of light in passing from one transparent medium into another. It is the quotient of the sine of the angle of the incident, divided by the sine of the angle of the refracted ray. In the case of oils which remain liquid at room temperatures, the determinations can be made without the aid of any device to maintain liquidity. In the case of fat which becomes solid at ordinary room temperatures, the determination must either be made in a room artificially warmed or the apparatus must have some device, as in the later instruments of Abbe and Pulfrich, and in the apparatus of Amagat-Jean, whereby the sample under examination can be maintained in a transparent condition. In each case the accuracy of the apparatus should be tested by pure water, the refractive index of which at 18° is 1.333. The refractive index is either read directly on the scale as in Abbe’s instrument, or calculated from the angles measured as in Pulfrich’s apparatus. [Illustration: FIG. 86.—ABBE’S REFRACTOMETER.] =296. Abbe’s Refractometer.=—For practical use the small instrument invented by Abbe will be found sufficient. The one which has been in use for many years in this laboratory is shown in Fig. 86. The illustration represents the apparatus in the position preliminary to reading the index. In preparing the sample of oil for observation the instrument is turned on its axis until the prisms between which the oil is placed assume a horizontal position, as is seen in Fig. 87. The movable prism is unfastened and laid aside, the fixed prism covered with a rectangular shaped piece of tissue paper on which one or two drops of the oil are placed. The movable prism is replaced in such a manner as to secure a complete separation of the two prisms by the film of oiled tissue paper. A little practice will enable the analyst to secure this result.