338 



NA TURE 



[February 12, 190; 



wave-length — an admirable triumph of mathematical 

 power. 



He proved (vol. i. p. 227) that the steepest possible 

 wave has a crest of 120 , with slope of 30° down from it 

 before and behind. He hoped to work out fully its 

 shape, and would no doubt have succeeded had time 

 permitted. 



Four short papers of July, 1845, February, 1S46, May, 

 1846, and July, 1846, 1 show that in those early times 

 Stokes had taken to heart the wave theory of light. 

 His later splendid work on light has given such great 

 results that even in the scientific world Stokes is often 

 thought of only as a worker in optics and the wave 

 theory of light. Truly his work in this province is more 

 than enough for the whole life-time of a hard-working 

 searcher in science. 



A short paper of great value,- " On the Formation of 

 the Central Spot of Newton's Rays beyond the Critical 

 Angle," touches in its title a physical question of funda- 

 mental importance — What motion takes place in the 

 ether close behind the per/vet mirror p?-esented by total 

 internal reflection f And the answer to it given in the 

 paper is admirably clear and satisfactory. 



A little later, we find one of the most important 

 of all of Stokes's papers on light, 3 "The Dynamica| 

 Theory of Diffraction." This paper contains the full 

 mathematical theory of the propagation of motion in a 

 homogeneous elastic medium. It contains, also, applica- 

 tion of the theory to the disturbance produced in ether by 

 a Fraunhofer grating for the two cases of incident light, 

 (1) with its vibrations in the plane of incidence, and (2) 

 with its vibrations perpendicular to that plane (therefore 

 parallel to the lines of the grating). Lastly, it contains a 

 description of an elaborate experimental investigation by 

 himself, and a comparison of the results with theory, from 

 which he concluded that the plane of polarisation is the 

 plane perpendicular to the direction of vibrations in plane 

 polarised light. This conclusion, notwithstanding adverse 

 criticism by Holtzmann, 4 was confirmed by Lorenz, of 

 Copenhagen."' The same conclusion was arrived at from 

 the dynamics of the blue sky by Stokes and Rayleigh, 

 and from the dynamics of reflection at the surface of a 

 transparent substance by Lorenz and Rayleigh. We 

 may now consider it one of the surest truths of physical 

 science. 



The greatest and most important of all the optical 

 papers of Stokes was communicated to the Royal Society 

 on May 27, 1852, under the title " On the Change of the 

 Refrangibility of Light." 11 In this paper, his now well- 

 known discovery of fluorescence is described ; according 

 to which a fluorescent substance emits in all directions 

 from the course through it, of a beam of homogeneous 

 light. The periods of analysed constituents of this 

 fluorescent light, in all Stokes's experiments, were found to 

 be longer than the period of the exciting incident light. 

 But I believe fluorescent light of shorter periods than 

 the exciting light has been discovered in later times. 

 Stokes found that the fluorescence vanished very 



1 M. and P. P., vol. i., pp. 141-157. 



- Camb. Phil. Soc, December 11, 184S, M. and P. P., pp. 56-81. 



:: Camb. Phil. Soc, November 26, 1849, M. and P.P.. pp. 243-328. 



4 Poggendorjff's Axnalt'n, vol. xcix., 1856, or Phil. Mag., vol. xiii. p. 135. 



s Poggendorjft's Annalen, vol. iii., r86o, or Phil. Mag., vol. xxi. p. 321. 



6 Phil. Trans, and M. and P.P., pp. 259-407. 



quickly after cessation of the incident light. A beautiful 

 supplement to his investigation was made by Edmond 

 Becquerel showing a persistence of the fluorescent light 

 for short times, to be measured in thousandths of a 

 second, after the cessation of the exciting light. 



Stokes's fundamental discovery of fluorescence is mani- 

 festly of the deepest significance in respect to the dyna- 

 mics of waves, and of intermolecular vibrations of ether 

 excited by waves, and causing fresh trains of waves to 

 travel through the fluorescent substance. The prismatic 

 analysis of the fluorescent light for any given period of 

 incident light was investigated by Stokes for a large 

 number of substances in his first great paper on the 

 subject, and was followed up by further investigations 

 by Stokes himself in later years, of which some of the 

 results are given in his piper " On the Long Spectrum of 

 the Electric Light " {Phil. 1 rans., June 19, 1862). 



Stokes's great paper on the refrangibility of light is 

 the last paper of the last volume (vol. iii.) hitherto pub- 

 lished ot his mathematical and physical papers. It is to 

 be hoped that with the least possible delay we shall have 

 a complete collected republication of all his other 

 papers. Every one of them, however small, will in all 

 probability be found to be a valuable contribution to 

 science ; witness, for example, his paper of twenty-one 

 lines in the Phil. Mag. for October, [872. Let us hope 

 that manuscript may be found for the communication to 

 the Royal Society promised at the end of that paper. 



Stokes's scientific work and scientific thought is but 

 partially represented by his published writings. He 

 gave generously and freely of his treasures to all who 

 were fortunate enough to have opportunity of receiving 

 from him. His teaching me the principles of solar and 

 stellar chemistry when we were walking about among 

 the colleges some time prior to 1852 (when I vacated my 

 Peterhouse fellowship to be no more in Cambridge for 

 many years) is but one example. Many authors of 

 communications to the Royal Society during the thirty 

 years of his secretaryship remember, I am sure grate- 

 fully, the helpful and inspiring influence of his con- 

 versations with them. I wish some of the students who 

 have followed his Lucasian lectures could publish to the 

 world his Opticae Lectiones j it would be a fitting sequel 

 to the "OpticiE Lectiones" of his predecessor in the 

 Lucasian chair, Newton. 



The world is poorer through his death, and we who 

 knew him feel the sorrow of bereavement. Kelvin. 



RECENT METHOD IN PRACTICAL 

 MA THE MA TICS. 



Hoherc Analysis Jiir Ingenieure. Von Dr. John Perry. 

 Autorisierte deutsche Bearbeitung von Dr. Robert 

 Fricke und Fritz Suchting. Pp. viii + 423. (Leipzig 

 und Berlin : Teubner, 1902). 



CONSIDERING the poor opinion the Germans 

 express for the school of mathematics in this country, 

 it is a great honour for Prof. Perry that his " Calculus for 

 Engineers " should be considered suitable for translation 

 as conveying a message of new method worthy of imitation 

 and adoption. 

 The improvement of the mathematical instruction 



NO. 1737. VOL. 67] 



