§ 38 .] MICROSCOPIC INVESTIGATION OF CRYSTALS. 6 l 
designated by the Greek letter oj, and that found by transmitting light 
parallel to the vertical axis by the Greek letter e. If oj is greater than 
e, the crystal is said to be optically negative ; if the reverse, positive. 
Or: 
co > e = negative. 
oj < e = positive. 
In the following table there is given a list of various alkaloids 
classified according to their optical sign. 
TABLE II. 
Positive Refraction. 
e a) 
Hydrastin 
• 
1-74-146 
+0-28 
Strychnine 
• 
1-73-1-63 
+0-10 
Cytisine . 
• 
1-73-1-64 
+0-09 
Quinidine 
• 
1-70-1-54 
+0-16 
Cinchonine 
• 
1-69-1-56 
+0-13 
Cinchonidine . 
a 
1-68-1-62 
+0-06 
Cinchonamine. 
• 
1-67-1-61 
+0-06 
Quinine . 
• 
1-58-1-57 
+001 
Aconite . 
• 
1-57-1-56 
+0-01 
Pierotoxin 
• 
1-56-1-54 
+0-02 
Negative Refraction. 
co e 
Thebaine 
• 
1-69-1-63 
-006 
Morphine 
• 
1-63-1-62 
-001 
Cupreine. 
• 
1-64-1-62 
-002 
Papaverine 
• 
1-68-1-62 
-006 
Atropine I 
Hyoscyamine J 
* 
• 
1-60-1-58 
-0-02 
Narceine. 
• 
1-61-1-57 
-0-04 
Codeine . 
• 
1-66-1-56 
-010 
Theine . 
• 
1-66-1-54 
-012 
Veratrine 
• 
• 
1-55-1-52 
-0-03 
Cantharidine . 
• 
1-56-1-52 
-0-04 
Theobromine . 
• 
1-74-1-54 
-0-20 
Cocaine . 
• 
1-62-1-50 
-012 
Narcotine 
• 
1-69-1-50 
-0-19 
Berberine 
• 
1-70-1-50 
-0-20 
Brucine . 
1-66-148 
-0-18 
To ascertain the two refractions the following method may be used. 
The condensing lenses are removed, a crystal is selected for observation, 
immersed in a liquid of known refractive index, and the nicols crossed 
so that the crystal extinguishes. The analysing nicol is now removed, 
and by raising the objective it is ascertained whether the crystal has a 
greater or less refraction than the immersing medium ; if it has not the 
