NATURAL PHILOSOPHY. 147 



blue, which color it assumes at a considerable angle of incidence. 

 AVhen the light reflected in the principal plane of the breadth is 

 examined in a similar manner, the pencil which is polarized in the 

 plane of incidence undergoes no remarkable change, continuing to 

 have the appearance of being reflected from a metaT, while the other 

 or colorless pencil vanishes at a certain angle and afterwards reap- 

 pears, so that in this plane the crystals have a polarizing angle. If, 

 then, for distinction's sake, AVC call the two pencils which the crystals, 

 as belonging to a doubly refracting medium, transmit independently 

 of each other, ordinary and extraordinary, the former being that 

 which is transmitted with little loss, we may say, speaking approxima- 

 tively, that the medium is transparent with respect to the ordinary 

 ray, and opaque with respect to the extraordinary ; while as regards 

 reflection, the crystals have the properties of a transparent medium or 

 of a metal according as the refracted ray is the ordinary or the extra- 

 ordinary. If common light merely be used, both refracted pencils 

 are produced, and the corresponding reflected pencils are mixed 

 together ; but by analyzing the reflected light, by means of a Xicol's 

 prism, the reflected pencils may be viewed separately, - - at least when 

 the observations are confined to the principal planes. The crystals arc 

 no doubt biaxal, and the pencils here called ordinary and extraor- 

 dinary are those which in the language of theory correspond to differ- 

 ent sheets of the wave surface. The reflecting properties of the 

 crystals ma}' be embraced in one view, by regarding the medium as 

 not only doubly refracting and doubly absorbing, but doubly metallic. 

 The ixietallicity, so to speak, of the medium of course alters continu- 

 ously with the point of the wave surface to which the pencil consid- 

 ered belongs, and doubtless is not mathematically null even for the 

 ordinary ray. If the reflection be really of a metallic nature, it ought 

 to produce a relative change in the phases of vibration of light polar- 

 ized in and perpendicularly to the plane of incidence. This conclu- 

 sion the author has verified by means of the effect produced on the 

 rings of calcareous spar. Since the crystals were too small for indi- 

 vidual examination in this experiment, the observation was made with 

 a mass of scales deposited on a flat black surface, and arranged at 

 random as regards the azimuth of their principal planes. The direc- 

 tion of the change is the same as in the case of a metal, and accord- 

 ingly the reverse of that which is observed in total internal reflection. 

 Iii the case of the extraordinary pencil the crystals are least opaque 

 with respect to red light, and accordingly they are less metallic with 

 respect to red light than to light of higher refrangibility. This is 

 shown by the green color of the reflected light when the crystals are 

 immersed in fluid ; so that the reflection which they exhibit as a trans- 

 parent medium is in a good measure destroyed. The author has 

 examined the crystals for a change of refrangibility, and found that 

 they do not exhibit it. Safflower red, which possesses metallic optical 

 properties, does change the refrangibility of a portion of the incident 

 light; but the yellowish green light which this substance reflects is 

 really due to its metallic-it}", and not to the change of refrangibility, 



