POLARIZATION OF LIGHT. 



603 



whether the mirror reflect the ray C E upwards, 

 downwards, sideways, or obliquely. No alteration, 

 in short, has taken place in the reflected light, ex- 

 cepting in the changes of its direction changes 

 which are accounted for on the ordinary laws of 

 optical science. But if to the extremity N of this 

 tube we attach another (fig. 2.) N M somewhat 

 greater in diameter, and affix to the end of M an- 

 other mirror A, so that a ray of light R A may fall 

 upon it from a luminous object R, at an angle of 

 incidence equal to 56, and this reflected ray pass 

 through the tubes in the direction A C, and be 

 reflected from the second mirror C to the eye at E, 

 also at an angle equal to 56; then will we find 

 that, by causing the mirror C to turn round on the 

 axis of the tube, some singular phenomena will take 

 place, not to be accounted tor by the ordinary prin- 

 ciples explanatory of the reflection of light. 



Fig. 2. 



These adjustments being made, place the apparatus 

 so that the plane of the mirror A is perpendicular 

 to the horizon, wherefore, the reflected ray A C will 

 be reflected in a horizontal direction, as represented 

 in the figure; then the tube N P being turned, so 

 that the ray C E, reflected from the mirror C, shall 

 1)e in the plane perpendicular to the horizon, it will 

 be found that the reflected image from the mirror 

 C, as seen at E, is very faint ; in fact, that, to all 

 appearance, there has been a great loss of light by 

 the reflection. Let the tube M N be in the same 

 position, as also the mirror A, but turn round the 

 smaller tube N P, until the reflected ray, from the 

 mirror C, is not, as formerly, perpendicular, but 

 horizontal, or, in other words, at right angles to the 

 plane of the ray C E, the tube N P having made 

 one quarter of a turn. During the turning of the 

 tube N P, it will be found that the reflected image 

 from the surface C, gets gradually more strong, 

 until the tube has been turned round one quarter 

 of an entire revolution from the position in which it 

 was first placed. Let the process of turning be con- 

 tinued until the smaller tube has performed a half 

 round ; the observer will find that from the begin- 

 ning to the end of this second quarter turn, the re- 

 flected image from the mirror C has gradually dimi- 

 nished in intensity, and that at the end it has be- 

 come as faint as it was at the beginning of the ex- 

 periment. Continuing the same process of turning 

 the smaller tube in the same direction, the intensity 

 of the image will increase from the beginning to 

 the end of the third quarter turn, when it will be 

 as bright as at the end of the first, and continuing 

 to turn the tube, the intensity will diminish until 

 the tube arrives at its first position, and the light 

 will be as dim as at the end of the second quarter 

 turn. We thus see that the reflected ray from the 

 reflector C is least intense when in the plane of the 

 perpendicular to the horizon, and most intense when 

 in the plane parallel to the horizon. 



In the course of this experiment, the same ray 

 A C has been reflected, but the intensity of the re- 

 flected ray varied with the position of the mirror C, 

 and since the luminous object R and the mirror A 

 have remained in the same position, it follows from 

 the forementioned results, that the upper and Hinder 



sides of the ray A C have scarcely been acted on at 

 all by the reflector C, and that the horizontal sides 

 have been acted upon more powerfully than any 

 other portions of the circumference of the ray. 

 Therefore, we infer that the ray A C has properties 

 entirely different from the ray of common light R 

 A, from which it has been obtained by reflection 

 from a glass mirror A, the plane of which is at an 

 angle of 56, with the incident ray R A of common 

 light. The inference to be drawn from this is. that 

 common light, when reflected from glass at an angle 

 of 56, acts more powerfully in one direction than 

 another, or, in other words, becomes polarized. 



The discovery of this singular property of light 

 is due to M. Malus, who, at an early period of lite 

 devoted his attention to the study of optical science, 

 j a study which was for a time interrupted by his 

 ~ service in the French army. In the summer of 

 1809, after his return from foreign service, he re- 

 sumed his favourite studies, and made frequent 

 visits to the observatory of Paris. In one of these 

 visits his attention was arrested by the brilliant re- 

 flection of the setting sun, from one of the windows 

 of Luxemburg palace. With a view of determining 

 the effect of double refraction (q. v.) on this reflected 

 light, he viewed it through a prism of quartz, and, 

 on turning it round, he was surprised to find that 

 one of the images resulting from the double refrac- 

 tion, changed gradually from brightness to obscu- 

 rity. He repeated the experiment under various 

 circumstances, and found that light reflected from 

 glass at a certain angle had properties similar to 

 the extraordinary ray in cases of double refraction. 

 Extending his observations, he found the polarizing 

 property in other reflecting surfaces, as water, mar- 

 ble, ebony, &c., each substance, however, having 

 a particular angle of incidence necessary to pola- 

 rization. 



This amiable and eminent philosopher was pro- 

 secuting his experiments with great success, when 

 death put an end to his brilliant career, in the prime 

 of life, by a lingering disease, in February, 1812. 

 The new field of science which Malus thus opened, 

 has been since farther explored by the philosophers 

 of our own country, as also of the continent. 

 Among the continental philosophers, who have 

 more particularly directed their labours to this sub- 

 ject, we may more especially mention M. M. Biot, 

 Arago, and Freznel, and among the men of science 

 of our own country, the names of Sir David Brew- 

 ster and Sir J. E. Herschel, stand deservedly con- 

 spicuous. Notwithstanding all the researches of 

 these able scientific men, a great deal yet remains 

 to be done ere the doctrine of the polarization of 

 light, as also the doctrine of double refraction, reach 

 that precision and simplicity which characterize 

 elementary optics. 



The knowledge hitherto obtained in this interest- 

 ing department of the science of light, is in a great 

 measure embodied in mathematical formulas, many 

 of which are complex, and altogether unsuited to a 

 work like the present. We will endeavour to make 

 the leading and more interesting results known tc 

 the reader in a more popular form than would be 

 pardonable in a systematic treatise on the subject. 



By observation the polarizing angles for many 

 substances have been found ; for example, the 

 polarizing angles of the undermentioned substance? 

 are as follow : 



Air, 45 or 47 



Water, 53 14 



Crown Glass, 56 12 



Rock Crystal, 57 22 



Iceland Spar, 58 51 



Glass of Antimony, 64 45 



Diamond, fi7 13 



