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]
| Name. | Temperature. | Refractive index. | Calculated 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 |
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].