302. Method of Charging the Apparatus.—The prism casing of the instrument is opened by turning the pin F to the right and pushing the half B of the prism casing aside. The prism and its appendages must be cleaned with the greatest care, the best means for this purpose being soft clean linen moistened with a little alcohol or ether.

Melt the sample of butter in a spoon and pour it upon a small paper filter held between the fingers and apply the first two or three drops of clear butter fat so obtained to the surface of the prism contained in prism casing B. For this purpose the apparatus should be raised with the left hand so as to place the prism surface in a horizontal position.

Press B against A and replace F by turning it in the opposite direction into its original position; thereby B is prevented from falling back and both prism surfaces are kept in close contact.

303. Method of Observation.—While looking into the telescope, give the mirror J such a position as to render the critical line which separates the bright left part of the field from the dark right part distinctly visible. It may also be necessary to move or turn the instrument about a little. First it will be necessary to ascertain whether the space between the prism surfaces be uniformly filled with butter, for, if not, the critical line will not be distinct.

By allowing a current of water of constant temperature to flow through the apparatus, some time previous to the taking of the reading, the at first somewhat hazy critical line approaches in a short time, generally after a minute, a fixed position and quickly attains its greatest distinctness. When this point has been reached note the appearance of the critical line (i. e., whether colorless or colored and in the latter case of what color); also note the position of the critical line on the centesimal scale, which admits of the tenth divisions being conveniently estimated, and at the same time read the thermometer. By making an extended series of successive readings and by employing an assistant for melting and preparing the small samples of butter, from twenty-five to thirty refractometric butter tests may, after a little practice, be made in an hour.

The readings of the refractive indices of a large number of butter samples made at 25° are, by means of a table which will be found below, directly reduced to scale divisions and yield the following equivalents:[254]

Natural butter(1.4590-1.4620) : 49.5-54.0scaledivisions.
Margarin(1.4650-1.4700) : 58.6-66.4””
Mixtures(1.4620-1.4690) : 54.0-64.8””

Whenever, in the refractometric examination of butter at a temperature of 25°, higher values than 54.0 are found for the critical lines these samples will, according to Wollny, by chemical analysis, always be found to be adulterated; but in all samples in which the value for the position of the critical line does not fall below 52.5, chemical analysis maybe dispensed with and the samples may be pronounced to be pure butter.

In calculating the position of the critical line for other temperatures than 25° allow for 1° variation of temperature a mean value of 0.55 scale division. The following table, which has been compiled in this manner, shows the values corresponding to various temperatures, each value being the upper limit of scale divisions admissible in pure butter:

Temp. Sc. div. Temp. Sc. div. Temp. Sc. div. Temp. Sc. div.
45°41.540°44.235°47.030°49.8
44°42.039°44.834°47.529°50.3
43°42.638°45.333°48.128°50.8
42°43.137°45.932°48.627°51.4
41°43.736°46.431°49.226°51.9
40°44.235°47.030°49.825°52.5