[ 3G6 ] 



LII. Further Researches into the Properties of the Sulphate of 

 Iodo-quinine or Herapathite, more especially in regard to its 

 Crystallography, ivith additional facts concerning its Optical 

 relations. By William Bird Herapath, 31. D* 

 [With a Plate.] 



AFTER referring to the observations of Professors Stokest and 

 Haidinger %, as well as to papers already published by him- 

 self on this subject in the Philosophical Magazine, the author gives 

 an account of a set of prisms perfectly complementary in their 

 optical characters to those previously described by him, and 

 proves this fact by an elaborate comparison of their various op- 

 tical relations ; from which it appeared, that whilst the a-prisms 

 (those described in the Philosophical Magazine for March 1852) 

 were totally impervious to a beam of polarized light, reflected 

 from glass plates, when the plane of the length of the prism was 

 at right angles to the plane of primitive polarization, the /3-prisins 

 (those now examined) were equally absorbent of a vertically 

 polarized beam, when the plane of their length lay parallel to 

 that of primitive polarization. See Plate III. diagram II. 

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

 Dissolve 10 grains of disulphate of quinine in half a fluid 

 ounce of spirit wine, having 3 grains of benzoic acid dissolved 

 in it also ; add 2 drachms of water and warm 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-brown or violet-red 

 if very thin ; in these respects they perfectly coincide with the 

 a-prisms. PI. III. diagram III. 



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. 

 PL III. diagram II. /3. 



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 complementary colours, viz. the green or yellow. 

 PL III. diagram II. fig. /3. 



* Reprinted from the Abstracts of the Papers communicated to the 

 Royal Society of London. 



t In a paper read at Belfast before the British Association (Athen. Sept. 

 1852) by Prof. Stokes. 



X In Poggendorff's Annalen, June 1853, and Phil. Mag. Oct. 1853, by 

 M. Haidinger. 



