224 J. J. FIELD ON THE RATIO-MICRO-POLARISCOPE. 



the prism can be set to its recorded reading instantly, whilst the 

 instrument remains " in situ," in fact, without any disturbance of 

 the general arrangements at all. 



Lastly, by means of this polariscope the elementary colours can 

 be mingled in any order or proportion that may be desired ; so that 

 any coloured field whatever, of absolute uniformity throughout, from 

 the dirtiest brown to the deepest and purest azure blue (even by 

 lamp light), may be produced instantly by making a known set- 

 ting. 



I had the pleasure of showing the instrument in action to our 

 President a few weeks back, and on producing, by an immediate 

 adjustment, this pure lovely blue of which I speak, he exclaimed, 

 " What a background for Guido's Angels !" Perhaps one of the 

 happiest expressions for mentally suggesting "the pure azure" 

 that could be devised. 



In now bringing the subject to a close, I will only add, that I 

 might say much as to the practical use of this polariscope and its 

 results ; but I abstain from any further remarks, feeling sure that 

 I must already have tired your patience to the fullest extent that I 

 can feel justified in doing. 



EXPLANATION OF PLATE X. 



> ♦ » • < 



Fig. 1. — Vertical View ; Polariscope with selenites locked in gear ready for 

 use in the microscope. 



Fig. 2. — Plan View j Polariscopo with selenites ungeared for making a 

 setting. 



