chap, xxx.] DOUBLE REFRACTION. 405 



of affairs interpose a plate of a doubly refractive 

 substance, for example, a plate of Iceland spar, and 

 let its principal plane be parallel to the first prism. 

 The ray from the first prism will be transmitted 

 unaffected by the plate, since their principal planes are 

 parallel, but will be extinguished by the second Nicol 

 since their planes are at right angles. Suppose next 

 that the plate is parallel to the second Nicol, that is, 

 is at right angles to the first Nicol. The plate, being 

 in its ordinary crystalline form, will transmit an 

 ordinary and an extraordinary ray, i.e. two rays 

 polarised at right angles to one another. The ray, 

 then, emerging from the first Nicol will not be ex- 

 tinguished by the plate because it can transmit rays 

 at right angles, but the second Nicol will extinguish 

 the ray because it can transmit rays only if vibrating 

 in its one plane, and not at right angles. But now 

 suppose the two Nicols still crossed, but the plate 

 interposed between them no longer parallel to either, 

 but with its principal plane forming an angle with 

 both, the lio'ht will now be transmitted through both 



o o 



Nicols. In short, if a plate of doubly refractive 

 material be interposed between the two crossed Nicols 

 in any position other than one in which its principal 

 plane coincides with that of either of the Nicols, light 

 will be enabled to pass through both Nicols. In other 

 words, if between two crossed Nicols, which con- 

 sequently appear dark, a substance be interposed 

 which makes the darkness give place to illumination, 

 however feeble, that substance is doubly refractive. 

 Hence there is supplied by a polarising apparatus a 

 test for discovering doubly refractive substances. 



How the doubly refractive plate can illuminate the 

 crossed Nicols, if forming an angle with both, may 

 be briefly indicated. Let NN 1 (Fig. 184) represent 

 the principal plane of the first Nicol, and N'-'N 2 the 

 principal plane of the second. They are at right 



