STOKES' "BURNETT LECTURES" 267 



The interest raised by the first series of these lectures is fully sustained by this 

 second instalment, though the subject-matter is of a very different order. Then, the 

 main question was the nature of light itself; now, we are led to deal chiefly with 

 the uses of light as an instrument for indirect exploration. It is one of the most 

 amazing results of modern science that the nature of mechanisms, too minute or too 

 distant to be studied directly with the help of the microscope or the telescope, can 

 be thus in part at least, revealed to reason. This depends on the fact that a ray 

 of light, like a human being, bears about with it indications alike of its origin and 

 of its history ; and can be made to tell whence it sprang and through what 

 vicissitudes it has passed. 



The lecturer begins by pointing out that this indirect use of light already forms 

 an extensive subject ; and he then specially selects for discussion half-a-dozen 

 important branches of it... 



The first of these is Absorption. Here we have the explanation of the colours 

 of bodies; the testing ray having gone in, and come out "shorn." This leads to 

 the application of the prism in the immediate discrimination of various solutions 

 which, to the unaided eye, appear to have the same colour. It is shown how, by 

 a mere glance, the chemist may often be saved from fruitless toil, occasionally from 

 grave error. 



From the study of what rays are absorbed, the transition is an easy and 

 natural one to the study of what becomes of them when they are absorbed. Here we 

 have heating, chemical changes, phosphorescence, etc. The remainder of the lecture 

 is devoted to an exceedingly interesting treatment of the beautiful subject of 

 fluorescence. 



The second lecture begins with Rotation of the Plane of Polarisation of light 

 by various liquids, with its important application to saccharimetry. Then we have 

 Faraday's discovery of the corresponding phenomenon produced in the magnetic 

 field, with its application in the discrimination of various classes of isometric 

 compounds 



Then comes the " still vexed " question of the history of Spectrum Analysis. 

 The present view of it must, of course, be carefully read : it is much too long to 

 be here extracted in full, and to condense would be to mutilate it. Of course 

 the claims of the author himself are the only ones to which scant justice is done. 

 But the President of the British Association of 1871 fortunately gave, in his opening 

 address, the means of filling this lacuna. Just as the Gravitation-theory of an 

 early Lucasian Professor was publicly taught in Edinburgh University before it 

 became familiar among scientific men, so the present Lucasian Professor's suggestions 

 for the analysis of the solar atmosphere, by means of the dark lines in the spectrum, 

 were publicly explained in the University of Glasgow for eight successive years before 

 the subject became generally known through the prompt and widespread publicity 

 given to the papers of Bunsen and Kirchhoff! 



The following are Sir William Thomson's words of 1871: "It is much to be 

 regretted that this great generalisation was not published to the world twenty years 

 ago... because we might now be (sic) in possession of the inconceivable riches of 

 astronomical results which we expect from the next ten years' investigation by 



342 



