For convenience the values of _n_ for all usual values of _i_ are computed once for all and arranged for use in tabular form. The latest model of Pulfrich’s apparatus, arranged both for liquid and solid bodies, and also for spectrometric observation is shown in Fig. 89. When the sodium light is used it is placed behind the apparatus and the light is collected and reflected on the refractive prism by the lens _N_. Through _H_ and _G_ is secured the micrometric reading of the angle on the scale _D_ by means of the telescopic arrangement _F E_. For regulating the temperature of the oil and adjacent parts, a stream of water at any desired temperature is made to circulate through _L_ and _S_ in the direction indicated by the arrows. The manner in which this is accomplished is shown in the cross section of that part of the apparatus as indicated in Fig. 90. [Illustration: FIG. 89.—PULFRICH’S NEW REFRACTOMETER.] [Illustration: FIG. 90.—HEATING APPARATUS FOR PULFRICH’S REFRACTOMETER.] [Illustration: FIG. 91.—SPECTROMETER ATTACHMENT.] For further details of the construction and operation of the apparatus the original description may be consulted.[248] In case a spectrometric observation is desired the _H_ ray, for instance, is produced by the geissler tube _Q_, Fig. 91. The light is concentrated and thrown upon the refractive prism by the lens _P_, the lens _N_, Fig. 89, being removed for this purpose. Tables, for correcting the dispersion and for calculating the indices for each angle and fraction thereof, and for corrections peculiar to the apparatus, accompany each instrument. =298. Refractive Indices of some Common Oils.=—The following numbers show the refractive indices obtained by Long for some of the more common oils. The light used was the yellow ray of the sodium flame.[249] Refractive Calculated Name. Temperature. index. for 25°. Olive oil (France) 26°.6 1.4673 1.4677 ” ” (California) 25°.4 1.4677 1.4678 Cottonseed oil 24°.8 1.4722 1.4721 ” ” 26°.3 1.4703 1.4709 ” ” 25°.3 1.4718 1.4719 Sesamé oil 24°.8 1.4728 1.4728 ” ” 26°.8 1.4710 1.4716 Castor ” 25°.4 1.4771 1.4773 Lard ” 27°.3 1.4657 1.4666 Peanut ” 25°.3 1.4696 1.4696