452 HISTORY OF SCIENCE. 



proved most fruitful of results as regards the theories, and the most 

 fertile as opening the way to new discoveries. In 1805 Malus was one 

 day viewing through a doubly-refracting crystal of Iceland spar the 

 rays of the sun reflected from the windows of the Luxembourg Palace. 

 Instead of two images, which with ordinary light are formed by Ice- 

 land spar, he was astonished to find that the crystal exhibited but 

 one. In whatever position he held the crystal, but one image of the 

 sun-illumined window could be seen ; the only difference being, that 

 in one position he perceived the image due to the " ordinary " ray, 

 and in the transverse position that due to the "extraordinary" ray. 

 In the evening of the same day on which he made this observation, he. 

 instituted some experiments in order to determine if possible the con- 

 ditions under which the phenomena were produced. He found that 

 the light of a candle reflected from a glass plate at one- particular angle 

 (about 35) was as completely polarized as the rays emerging from 

 crystals of Iceland spar. A ray reflected from water at the incidence 

 of 36 he found also to be polarized ; and, in general, the light re- 

 flected from all transparent bodies is always more or less polarized, 

 and that reflected at a certain angle which varies from one body to 

 another is completely polarized. Here, then, was a curious and but 

 recently discovered property of light, hitherto known only as an ex- 

 ceptional and rare phenomenon connected with only a very few kinds 

 of mineral crystals, at once proved to be as common as daylight and 

 as ancient as the world. At first Malus supposed that, besides the 

 polarization by doubly-refracting crystals, reflection was the only means 

 of polarizing light. But further investigations showed that the light 

 which had passed through transparent bodies was, to some extent, 

 polarized. The fact is, that if a ray of ordinary light is incident on a 

 plate of glass at the angle of 35, a portion of the light is reflected, and 

 the whole of this portion is polarized ; but the greater part is trans- 

 mitted, and of this transmitted beam a part is polarized equal to that 

 which is polarized by reflection. By increasing the number of succes- 

 sive reflections, we can obtain a ray almost completely polarized by its 

 passage through a sufficient number of parallel transparent plates. 

 Hence a bundle of thin plates of glass is often used as the means of 

 obtaining polarized light. 



Malus himself was an adherent of the Newtonian or corpuscular 

 theory of light, and his study of polarization phenomena led him to 

 conclude that neither the corpuscular nor the undulatory theory could 

 explain these phenomena. Even Young declared that their explana- 

 tion was a problem which " would probably long remain to mortify 

 the vanity of an ambitious philosophy, completely unresolved by any 

 theory." But though admitting the insufficiency of his own hypotheses 

 respecting the nature of light to explain the phenomena of polarization, 

 yet he did not consider that these phenomena had proved the undula- 

 tory theory to be false. These admissions were welcome intelligence to 



