APPENDIX. 503 



pigment cells ; but this is not correct, as Mr. De la Rue has 

 convinced himself by careful and repeated examinations, more 

 especially from the under side, that the stri» themselves are 

 really beaded; it is true that the pigment-cells correspond 

 very exactly in position with the striae, v?hich is very remark- 

 able ; but, in some of the deeply coloured scales, there is a 

 granulated appearance covering the entire surface of the scale 

 very uniformly, in vsrhich the constricted appearance of the 

 strife is even more apparent. Hence it vrould appear that the 

 peculiarity in the markings of the Amathusia Horsfieldii is 

 due, not to their consisting of minute scales, but to the 

 superposition of pigment-cells exactly over the beaded striae ; 

 the transverse markings exhibiting the appearance most 

 strongly." 



NEW TEST OBJECTS. 



Since the last edition of the present work was pubHshed, 

 two test objects, having extremely close and shallow undulating 

 surfaces, have been employed by microscopic observers in- 

 terested in this branch of inquiry. The first of these is 

 from America, and is one of the Diatomaceae termed Gram- 

 matophora subtilissima, whilst the other is a Navicula, first 

 employed by Amici. The utmost management with oblique 

 illumination, and its precise direction with reference to the 

 striae, is necessary for their exhibition ; the observer must be 

 prepared with plenty of patience, and provided with glasses of 

 very large aperture, to recognise the striations. Mr. Ross's 

 object-glasses of j?^ of an inch focal length and 152 degrees 

 of angular aperture, exhibit them satisfactorily. 



THE PRODUCTION OF ABTIFICIAL POLARIZING CRYSTALS. 



By dissolving disulphate of quinine in acetic acid, and care- 

 fully dropping into the solution a dilute spirituous solution of 

 iodine, warming the mixture so as to redissolve the precipitate 

 first formed, a compound is produced which has the remarkable 



