22 Mr. R. H. M. Bosanquet on a new Form of Polariscope, 



specimens procured in Switzerland. The crystals proved to 

 be full of striae ; but notwithstanding this, the appearance of 

 the bands is substantially perfect when the line of vision co- 

 incides pretty closely with the axis. This is an observation 

 worth noting- ; for it seems probable that most of the Swiss 

 quartz is so striated as to make it useless for many purposes, 

 but that the striated structure is so situated as not to affect 

 vision in the direction of the axis. I possess some fine Swiss 

 crystals with well-developed hemihedral faces, which present 

 external markings similar to those on the crystals which 

 proved striated : if such as these can be used for the present 

 purpose, the expense of perfect crystals of the requisite size 

 may be avoided ; for any one can pick up in Switzerland for 

 a few francs large and fine specimens of this kind. 



The small instrument of Swiss quartz is mounted at the end 

 of a tube 10 inches long with a small Nicol's prism interposed 

 between it and the eye. It gives, with polarized mean yellow 

 light, three complete bands, and already forms a very useful 

 and sensitive polariscope for ordinary purposes. 



By reducing to millimetres, taking the rotation of the mean 

 yellow ray at 24° for 1 millim., and remembering that a 

 parallelopiped of this kind made up of two opposite quartz 

 wedges exerts a rotatory effect due to twice its depth, we can 

 show that every complete band in the mean yellow requires 

 a depth of *15 of an inch from back to front. To get a more 

 extensive series of bands I procured two fine pieces of quartz, 

 right-handed and left-handed respectively, out of which the 

 compound quartz first described, viz. 1*5 inch long and # 9 inch 

 in section, was constructed. According to the above calcula- 

 tion this should give ten bands with yellow polarized light. 

 A rather small stop has been inserted. But within this I can 

 count eight bands when the instrument is directed to the re- 

 flection of a salted spirit-flame in a polished table. Of course 

 with white light the systems of bands belonging to the dif- 

 ferent colours are soon superposed, and only about five or six 

 bands are prominent. But the central bands, whether white 

 or black, are sharply defined. 



The bands of this compound quartz are delicate and beautiful 

 when it is held at some distance and looked at through a Nicol 

 with polarized light ; but it was my object to secure sensitive- 

 ness by bringing it nearer the eye so as to get more light ; 

 and also it was necessary to have it mounted with an adjust- 

 ment, by which the axis could be placed parallel to the line of 

 vision. The form adopted is illustrated in the accompanying 

 figure. Parallel rays, after traversing the axis of the com- 

 pound quartz, fall upon a simple lens of 4 inches focal length 



