62 POISONS : THEIR EFFECTS AND DETECTION. [§ 39. 
same, one liquid or mixture of liquids is tried successively until the 
desired result is obtained, the position of the crystal not being 
altered. 
The graduated circle on the revolving stage is now read and noted ; 
the stage is now rotated exactly 90° to its position of extinction and the 
process repeated ; these two values will be the two indices required. 
Very often there is quite a coarse difference, for example :— 
a) 
£ 
Difference. 
Theobromine . 
1-74 
1-54 
-0-20 
Theine .... 
1-66 
1-54 
-0-12 
Brucine .... 
1-66 
1-48 
-0-18 
Strychnine 
1-63 
1-73 
+010 
These minus or plus values obviously assist identification when 
present, but in many cases, e.g. atropine, narceine and aconitine, the two 
refractive indices are too similar to be of use. 
With regard to biaxial crystals, it is seldom practicable to determine 
three indices, nor can the determination of these indices settle the 
positive or negative value of the crystal. 
The axis of maximum rate of transmission is usually designated by 
the German letter a, the minimum by r, the intermediate by b. A 
maximum and minimum value of a biaxial crystal may be found, by 
adjusting a crystal in a liquid of known refractive index in the 
position of extinction ; then in this position, by substituting one 
liquid or mixture of liquids, ascertain the refractive index: the 
crystal is then rotated exactly 90° from the extinction position, 
and the refractive index again ascertained. By operating in this 
way on several crystals, a maximum and a minimum refraction can 
with fair accuracy be obtained. The chief practical difficulty will 
be found in keeping the crystal under observation from shifting 
its position during the necessary manipulations. 
VIII.—The Spectroscope as an Aid to the Identification 
of certain Poisons. 
§ 39. The spectra of many of the metals, of phosphine, of arsine, 
and of several other inorganic substances, are characteristic and easily 
obtained. 
It is, however, from the employment of the micro-spectroscope that 
the toxicologist is likely to get most assistance. 
