CHAP. V., 3.] 



OPTICS. FRESNEL. 



105 



484.) 



bone. He does not appear to have noticed the phe- 

 nomena of colour accompanying such depolarization, 

 though he arrived at it so nearly that Arago, in all pro- 

 bability, anticipated him by presenting a memoir on 

 the subject to the Institute just one week previous to 

 Malus' s announcement of what he had observed 

 (19th August 1811). In less than six months later 

 Malus was no more. 



The writings and discoveries of Malus present evi- 



dence of great talent, but of far less fertility of com- Character 

 bination than those of Fresnel, presently to be no- of Malu8 - 

 ticed. He maintained the Newtonian theory of light. 

 His reputation amongst his intimates was extremely 

 high, and it was generally believed, that had he sur- 

 vived, his discoveries would have extended much far- 

 ther. To him was applied Newton's saying on the 

 death of Cotes, " If Cotes had lived we should have 

 learned something." 



3. FRESNEL. The Undulatory Theory of Light continued. Diffraction. Transverse Vibra- 

 tions ; Young. Polarization and Double Refraction explained. Lighthouse Illumination. 



AUGUSTIN FRESNEL was born at Broglie, in France, 

 10th May 1788, of a feeble constitution, and he con- 

 tinued throughout his too short life a prey to attacks 

 of bad health. As a boy, his slow apprehension and 

 uncertain memory gave no indication of the maturity 

 of his judgment. He entered first the Polytechnic 

 School, then that of Fonts et Chaussees. His fidelity 

 to the Bourbon cause occasioned his being harshly 

 treated by Napoleon, and he retired to Normandy in 

 the beginning of 1815, to pursue the scientific studies 

 which he had always loved. 



Di/racnon. The theory of light in particular at- 

 light. tracted his attention, and he had a steady belief that 

 the Newtonian doctrine was erroneous, though in 

 ignorance, as it appears, of the undulatory doctrines 

 of Hooke, Huygens, and Young. The phenomena 

 of diffraction, or the coloured fringes which are seen 

 in the interior of the shadows of opake bodies when 

 illuminated by a minute source of light, attracted 

 his attention as most proper for deciding the deli- 

 cate question of the molecular or undulatory cha- 

 nnel's racter of light. The results of his experiments were 

 detailed in a memoir confided in the first place to 

 his friend Arago, and by him communicated to the 

 Institute of France (October 1815). This remark- 

 able paper contained much which Dr Young had al- 

 ready discovered, and the explanations of the experi- 

 ments which it described, both new and old, by the 

 theory of undulations, were common to both. Dr 

 Young having anticipated the publication by at least 

 a dozen years, there could be no question of pri- 

 ority ; but it is equally certain that Fresnel was un- 

 aware of what Young had done until it was pointed 

 out to him by Arago. His memoir, which was pub- 

 lished in great part in the Annales de Chimie for 

 1816. contains much which is interesting. The mode 

 of observing the diffraction bands directly by means 

 of a lens, without the intervention of a screen, was 

 equally new and important. The observation that 

 the interior fringes of the shadow of a narrow body, 

 such as a wire, disappear when the light is intercepted 

 on either side of the wire, leading to the conclusion 

 that the union of the light from both sides is neces- 



t me 



ir. 



sary for their occurrence, was (as we have seen) 

 one of Young's capital experiments. The explana- 

 tion of Newton's rings, by the interference of the light 

 reflected from two adjacent surfaces, though partly 

 anticipated by Hooke, was equally important. Nu- 

 merous measures of the distances of the exterior 

 diffraction bands from the geometrical shadow, as 

 formed by homogeneous red light, are then given and 

 compared with theory. Here Fresnel was on original 

 ground. These accurate numerical comparisons, af- 

 terwards pursued to a greater extent, constituted one 

 of the most important bases of the new theory. In 

 obtaining them he was materially aided by Arago, 

 who, though considerably his senior, generously as- 

 sisted him in every respect, and gave him the full ad- 

 vantage of his station as a member of the Institute, 

 and of his experience. 



Fresnel's first memoir on diffraction justly excited (487.) 



so much notice that the subject was proposed by the Sec . ond me * 

 A j & ct inii-r /> p ., . . moir. Im- 



Acaaemy ot bciences in 1817 tor one or their prizes. proves 



The new essay which Fresnel then wrote was, as pro- upon 

 bably had been anticipated, the successful one. In Young's 

 this memoir he made an important step, by showing 

 that the exterior fringes in diffraction shadows do not 

 depend (as Young had supposed) upon the interfer- 

 ence of the direct light with that reflected at a great 

 obliquity from the edge of the diffracting body, but 

 from the interference of the different elementary un- 

 dulations which proceed from the disturbed surface 

 forming the front of the grand wave. Decomposing 

 the front of the wave into small portions after the 

 manner of Huygens, he computed the disturbance 

 produced by the integral effect of the whole at a given 

 point of the screen where the picture of the shadow 

 fell and was submitted to examination, and he fouud 

 that such integral effects have a periodic character, 

 presenting points of maximum and minimum distur- 

 bance, or of greatest and least illumination as we re- 

 cede from the geometrical shadow. These distances 

 being measured in homogeneous red light were found 

 to agree with the results of an arduous computation, 

 requiring, as will easily be seen, an intimate acquain- 

 tance with the integral calculus and much skill in 



