XX111 



rank among the cultivators of optical science, and in the year 1833 he 

 was requested by the British Association to report on the condition of 

 physical optics. The report prepared in compliance with this request 

 was laid before the British Association in the year 1834, and may be 

 regarded as a handbook of the progress of the science to that date. 



Shortly after the publication of the experiment which established 

 the reality of conical refraction, Dr. Lloyd described to the Royal 

 Irish Academy an important experiment upon the interference of 

 light proceeding directly from a luminous source with light coming 

 from the same source, but reflected at a very high angle of incidence 

 from a plane surface. By means of this experiment he was able to 

 make an important contribution to the theory of reflected light. The 

 phenomena of thin plates require us to admit that a semi-undulation 

 is gained or lost by the light in the process of reflexion at one of the 

 surfaces. But these phenomena do not decide the question whether 

 this modification takes place at the surface of the rarer or of the 

 denser medium. Dr. Young had given the preference to the former 

 of these alternatives ; but Dr. Lloyd derived from the above-mentioned 

 experiment a strong argument in favour of the other. The details of 

 this experiment are published in the seventeenth volume of the 

 " Transactions of the Royal Irish Academy." 



In 1836 Dr. Lloyd published the first part of his lectures on the 

 "Wave Theory of Light," including the phenomena which are inde- 

 pendent of polarisation and double refraction. To this was subsequently 

 added a second part in which the phenomena of polarisation are dis- 

 cussed. 



A communication received from Sir David Brewster, detailing some 

 remarkable appearances which he had observed in connexion with the 

 phenomena of thin plates, induced Dr. Lloyd to turn his attention to 

 that subject, the light incident on the plate being supposed to be 

 polarised. A communication on this subject was made by him to the 

 British Association in 1841, but the complete investigation of the 

 phenomenon was published in the twenty-fourth volume of the " Trans- 

 actions of the Royal Irish Academy," having been laid before that 

 Society in 1859. 



Assuming the truth of Fresnel's expressions for the intensity and 

 phase of polarised light reflected from the surface of an ordinary 

 medium, Dr. Lloyd showed that the reflected light is elliptical]y 

 polarised. He assigned the law of this elliptic polarisation, which 

 passes into plane polarisation where the incident light is polarised in, 

 or at right angles to, the plane of incidence. He also gave the 

 explanation of the phenomena observed by Brewster, where the index 

 of refraction of the plate is intermediate between those of the bound- 

 ing media. 



But it will probably be felt that the link which associates Dr. Lloyd's 



