J. J. FIELD ON THE RATIO-MICRO-POLARISCOrE. 219 



Land so great as to be nearly, but not quite equal, to a semi-undula- 

 tion ; interference might take place between the two, without, in the 

 first case, any perceptible diminution of luminosity ; and in both 

 cases without any perceptible production of colour. 



To begin with the violet end of the spectrum : let us assume 

 that a polarised ray, having been split by passage through any 

 doubly refracting substance into the ordinary and extraordinary 

 rays, the former is in advance of the latter — as to phase — by such 

 a distance as exactly corresponds to a semi- undulation of the most 

 refrangible or chemical rays ; then it is clear that if these two 

 beams were brought together by the influence of an analysing 

 prism, the only portions that would be extinguished by such clash- 

 ing, would be these same chemical rays, and under the conditions 

 named, they would absolutely disappear. 



Next, to go to the other end of the spectrum, and deal with the 

 least refrangible, or calorific rays : if the structure viewed were of 

 so much greater thickness as to make a difference in phase equal to 

 a semi-undulation of the heat rays, then these alone would be 

 totally suppressed; and although, in this latter case, the entire 

 beam would also suffer as to luminosity — for a reason that I need 

 not here enter upon — still no chromatic effects would be produced 

 whatever. 



To take the third case, I will, for simplicity, assume that the 

 particular structure is capable of retarding the extraordinary ray 

 by a half undulation of yellow light : now when the two beams or 

 rays are brought together by the action of the analyser, the yellow 

 will be struck from the spectrum, and the object will appear 

 arrayed in the complimentary tint. 



It is evident that a similar result must occur when the phase- 

 displacement corresponds to a semi- undulation of any chromatic 

 ray ; that ray must at once disappear, and leave such a balance of 

 colour as shall be its complement in relation to white light. 



If in this description I have been sufficiently clear, the idea I 

 wish to advance, as to the action of the selenites* will now be 

 understood. 



I apprehend that in the case of a great number of doubly-refract- 



* Not in the production of colour 'per se, but in rendering evident tlie pro- 

 perty of double refraction in details of the structure that before seemed not to 

 possess it. 



