399 



sorbent of a vertically polarized beam, when the plane of tbeir length 



lay parallel to that of primitive polarization. 



These prisms may be readily made by the following process : — 

 Dissolve 10 grains of disulphate of quinine in half a fluid ounce 



of spirit of wine, having 3 grains of benzoic acid dissolved in it also ; 



adding 2 drachms of water and warming the whole to complete 



solution, then upon adding a few drops of spirituous solution of 



iodine and placing in repose, prismatic crystals having the following 



properties are produced : — 



1 . When two are crossed in superposition at right angles to each 

 other or even at 30°, the overlapped space is " black " if the crystals 

 are sufficiently thick, but reddish browTi or violet- red if very thin ; 

 in these respects they perfectly coincide with the a-prisms. 



2. When examined by polarized light reflected from glass, the 

 vertical crystals now appear black, whilst the horizontal prisms are 

 almost perfectly transparent to the polarized beam. 



3. Upon introducing the selenite stages beneath the prisms, those 

 which previously obstructed the polarized beam and appeared "black," 

 transmitted the "red" or "blue" colours of the selenite films, 

 whilst the transparent, horizontal prisms, exhibited the comple- 

 mentary colours, viz. the green or yellow. 



The last two completely decided their complementary character, 

 for whilst the /3-prisms were "black," "red " or " blue," when the 

 planes of their length were vertical, the a-prisms were " black," 

 " red " or " blue " whilst the planes of their length were horizontally 

 placed. 



4. But by mixing the two prisms upon one slide their perfectly 

 complementary character was completely demonstrated, for when an 

 a-prism was crossed at right angles by a /3-prism, the square space 

 where they overlapped was still transparent to common light; but if 

 a /3-prism were overlapped by an a-prism, the length of the two being 

 parallel to each other, the oblong space where they overlapped be- 

 came black from their doubly absorbent properties ; in short, the 

 optical examination fully proved the complementary character of these 

 crystals length for length. 



It was further shown that two varieties of hexagons existed ; those 

 produced from strong spirituous solutions were longer than broad, 

 and possessed two acute angles of 65^ and four equal angles of 

 147° 30'. Examined by a vertically plane- polarized beam, these 

 were "black" when a line passing through the long diameter or 

 acute angles was parallel to the plane of primitive polarization : these 

 the author called " acute " or /3-hexagons. The hexagons produced 

 from acetic acid solutions, on the other hand, were " obtuse " in their 

 character, and had two angles of 115° and four equal angles of 

 122° 30'. These, when examined by vertically plane-polarized light, 

 were always " black " when a line passing through the two angles 

 of 11.5° was perpendicular to the plane of primitive polarization : the 

 author called these " obtuse " or a-hexagons. 



The rhombic crystals which presented themselves were also "black" 

 (examined in the same way) when their long diameters lay parallel 



