178 
BRITISH PHARMACEUTICAL CONFERENCE. 
or system of tests at present used are free from objection. An inspection of 
tlie optical characters of the oils, whether fixed or volatile, will be sufficient to 
confirm the truth of this observation. 
The polariscope has at best a very limited scope of application, whilst the de¬ 
termination of the refractive or dispersive qualities requires such precise ad¬ 
justments that the suitability, either of the one or the other, for the purposes of 
a technical test may be fairly questioned. The refractive power of the oils, 
both fixed and volatile, has so small a variation, even between the two extremes 
of the scale here given, that the difference produced on the refractive power of 
any oil by the addition of a small quantity of another, would be barely per¬ 
ceptible. The objection against the measurement of the dispersive action as a 
means of expressing the value of an oil, is that the determination of the dif¬ 
ferences of indices of refraction for the extreme rays is at once tedious and un¬ 
reliable ; the scale of dispersions offers, however, a much wider range of 
differences. 
It is probable that the comparison of two samples of oil by the irrationalities 
of their dispersion is worthy of some attention. I am not aware of its being 
applied as a test, but the samples could stand side by side with respect to the 
illuminating source, and their spectra projected side by side could be easily ob¬ 
served and compared. 
The tables here given of the dispersive powers, and the irrationalities of dis¬ 
persions, are by Sir David Brewster; the refractive powers are principally by 
the same authority. 
Optical Qualities of Oils. 
Name of Oil. 
Index of 
Refraction. 
Dispersive 
Power. 
Difference of 
Index of 
Refraction 
for Extreme 
Rays. 
Anise ....... 
1-601 
•077 
•044 
Almond bitter .... 
1-603 
•079 
•048 
Almond sweet .... 
1-483 
Angelica. 
1-493 
•051 
•025 
Bergamot ..... 
1-471 
Cassia. 
1-641 
•139 
•089 
Caraway ..... 
1-491 
•049 
•024 
Castor. 
1-490 
•036 
•018 
Camomile. 
1-457 
Cloves . 
1-535 
•062 
•033 
Cumin. 
1-508 
•065 
•033 
Dill. 
1-477 
Fennel. 
1-506 
•055 
•028 
Juniper. 
1-473 
•047 
•022 
Lemon. 
1-476 
Lavender ..... 
1-462 
Nutmeg ...... 
1-497 
Olive. 
1-470 
•038 
•018 
Poppy . 
1-463 
Pennyroyal. 
1-482 
Peppermint. 
Eape-seed. 
1-475 
Sassafras . 
1-534 
•069 
•032 
Spearmint. 
1-481 
•054 
•026 
Alcohol. 
1-372 
•029 
•Oil 
Turpentine. 
1-475 
•042 
•020 
