In case of the use of Abbe’s apparatus, in which diffused sunlight is the source of the illumination, the numbers obtained cannot be compared directly with those just given unless the apparatus be first so adjusted as to read with distilled water at 18°, 1.333. In this case the reading of the scale gives the index as determined by the yellow ray. The numbers obtained with Abbe’s instrument for some common oils are given below.[250] In the determinations the instrument was set with water at 18°, reading 1.3300, and they were corrected by adding 0.0030 in order to compensate for the error of the apparatus. Material. Calculated for 25°. Corrected index. Lard 1.4620 1.4650 Cotton oil 1.4674 1.4704 Olive oil stearin 1.4582 1.4610 Lard stearin 1.4594 1.4624 =299. Oleorefractometer.=—Instead of measuring the angular value of the refractive power of an oil it may be compared with some standard on a purely arbitrary scale. Such an apparatus is illustrated by the oleorefractometer of Amagat-Jean, or by Zeiss’s butyrorefractometer. In the first named instrument, Fig. 92, the oil to be examined is compared directly with another typical oil and the shadow produced by the difference in refraction is located on a scale read by a telescope and graduated for two different temperatures.[251] The internal structure of the apparatus is shown in Fig. 93. [Illustration: FIG. 92.—OLEOREFRACTOMETER.] [Illustration: FIG. 93.—SECTION SHOWING CONSTRUCTION OF OLEOREFRACTOMETER.]