After the paper disk holding the fat is secured by replacing the upper prism, the apparatus is placed in its normal position and the index moved until the light directed through the apparatus by the mirror shows the field of vision divided into dark and light portions. The dispersion apparatus is now turned until the rainbow colors on the part between the dark and light fields have disappeared. Before doing this, however, the telescope, the eyepiece of the apparatus, is so adjusted as to bring the cross lines of the field of vision distinctly into focus. The index of the apparatus is now moved back and forth until the line of the two fields of vision falls exactly at the intersection of the cross lines. The refractive index of the fat under examination is then read directly upon the scale by means of a small magnifying glass. To check the accuracy of the first reading, the dispersion apparatus should be turned through an angle of 180° until the colors have again disappeared, and, after adjustment, the scale of the instrument again read. These two readings should nearly coincide, and their mean is the true reading of the fat under examination. [Illustration: FIG. 87.—CHARGING POSITION OF REFRACTOMETER.] For butter fats the apparatus should be kept in a warm place, the temperature of which does not fall below 30°. For reducing the results obtained to a standard temperature, say 25°, the factor 0.000176 may be used. As the temperature rises the refractive index falls. _Example._—Refractive index of a butter fat determined at 32°.4 = 1.4540, reduced to 25° as follows: 32.4 -25 = 7.4; 0.000176 × 7.4 = 0.0013; then 1.4540 + 0.0013 = 1.4553. The instrument used should be set with distilled water at 18°, the theoretical refractive index of water at that temperature being 1.333. In the determination above given, the refractive index of pure water measured 1.3300; hence the above numbers should be corrected for theory by the addition of 0.0030, making the corrected index of the butter fat mentioned at the temperature given, 1.4583. =297. Pulfrich’s Refractometer.=—For exact scientific measurements, Pulfrich’s apparatus has given here entire satisfaction. In this instrument a larger quantity of the oil is required than for the abbe, and this quantity is held in a cylindrical glass vessel luted to the top of the prism. The method of accomplishing this and also an illustration of the refraction of the rays of light are shown in Fig. 88. [Illustration: FIG. 88.—PRISM OF PULFRICH’S REFRACTOMETER.] The angle _i_ is measured by a divided circle read with the aid of a small telescope. The index of the prism of highly refractive glass _N_ is known. The oil is seen at _n_. The light used is the yellow sodium ray (_D_). From the observed angle the refractive index of n is calculated from the formula ______________ _n_ = √_N_² - sin²_i_.