SIX LECTURES ON LIGHT. 



27 



being thus polarized, if they be suitably re- 

 ceived upon a plate of glass at the polarizing 

 angle, one of them will be reflected, the 

 other not. This is the conclusion of reason 

 from our previous knowledge; but you ob- 

 serve that reason is justified by experiment. 

 (Figs. 15 and 16.) 



I have said that the whole of the beam re- 

 flected from glass at the polarizing angle is 

 polarized; a word must now be added regard- 

 ing the larger portion of the light transmitted 

 by the glass. The transmitted beam contains 

 a quantity of polarized light equal to that of 

 tie reflected beam; but this quantity is only 

 a fi action of the whole transmitted light. By 

 taking two plates of glass instead cf one, we 



(B is the birefracting spar, dividing the incident 

 li^fit into the two beams, o and e. G is the mirror). 

 Tne beam is here reflected laterally. When the re- 

 flection is ufitvards^ the other beam is reflected, as 

 shown in Fig. 16. 



augment the quantity of the transmitted polar- 

 ized light; and, by taking a bundle of plates, we 



so increase the quantity as to render the trans- 

 mitted beam, for all practical purposes, per- 

 fectly polarized. Indeed, bundles of glass 

 plates are often cmp'oyed as a means of fur- 

 nishing polarized light. 



One word more. When the tourmalines 

 are crossed, the space where they cross each 

 other is black. But we have seen that the 

 least obliquity on the part of the crystals per- 

 mits light to get through both. Now sup- 

 I ose, when the two plates are crossed, that 

 we interpose a third plate of tourmaline be- 

 tween them, wiih its axis oblique to both. A 

 portion of the light transmitted by the first 

 plate will get through this intermediate cne. 

 But, after it has got through, its plane of vi- 

 bration is changed: it is no longer perpendicu- 

 lar to the axis cf the crystal in front. Hence 

 it will get thiough that crystal. Thus, by 

 reasoning, we infer that the interposition of a 

 third plate of tourmaline will in part abolish 

 the darkness produced by the perpendicular 

 crossing of the other two plates. I have not 

 a third plate of tourmaline ; but the talc or 

 mica which you employ in your stoves is a 

 more convenient substance, which acts in the 

 same way. Between the crossed tourmalines 

 I introduce a film of this crystal. You see 

 the edge of the film slowly descending, and 

 as it descends between the tourmalines, light 

 takes the place cf darkness. The darkness, 

 in factj setmed scraped away as if it were 

 something material. This effect has been 

 called and improperly called depolarization. 



LECTURE IV. 



Chromatic Phenomena produced by Crystals on Polar- 

 ized Light: The Nicol Prism : Polarizer and Ana- 

 lyzer: Action of thick and thin Plates of Selenite: 

 Colors dependent, on Thickness: Resolution of Po- 

 larized Beam into two others by the Selenite ; One 

 of them more retarded than the other: Recom- 

 pounding of the two Systems of Waves by the Ana-r 

 lyzer: Interference thus rendered possible : Conse- 

 quent Production of Colors: Action of Bodies 

 Mechanically strained or pressed : Action of Sono- 

 rous Vibrations: Action of Glass strained or pressed 

 by Heat: Circular Polarization: Chromatic Phe- 

 nomena produced by Quartz : The Magnetization 

 of Light: Rings surrounding the Axes of Crystals: 

 Blaxal and Uniaxal Crystals : Grasp of the Undu- 

 latory Theory. 



We now stand upon the threshold of a new 

 and splendid optical domain. We have to 

 examine, this evening, the chromatic phe- 

 nomena produced by the action ot crystals, 

 and double-refracting bodies gene: ally, upon 

 polarized light. For a long time investigators 

 were compelled to employ plates of tourmaline 

 for this purpose, and the progress they made 

 with so defective a means of inquiry is aston- 

 ishing. But these men had their hearts in 

 their work, and were on this account enabled 

 to extract great results from small instrumen- 

 tal appliances. But we have better apparatus 

 now. You have seen the two beams emer- 

 gent from Iceland spar, and have proved 

 them to be polarized. If we could abolish 



