744 THE POPULAR SCIENCE MONTHLY. 



where progress demands tliat tliese distinct and rarely associated pow- 

 ers be brought simultaneously into action. For there the mathema- 

 tician has not merely to save the experimenter from the fruitless labor 

 of pushing his inquiries in directions where he can be sure that (by 

 the processes employed) nothing new is to be leai'ned ; he has also to 

 guide him to the exact place at which new knowledge is felt to be 

 both -necessary and attainable. It is on this account that few men 

 have ever had so small a percentage of barren work, whether mathe- 

 matical or experimental, as Stokes." 



A partial list of Stokes's contributions to science is given in Prof. 

 Tait's memoir. It is there stated that up to 1864 Stokes had pub- 

 lished the results of some seventy distinct investigations. Since that 

 year he has published but little, though it is well known that he has 

 in retentis several optical and other papers of the very highest order, 

 which he cannot bring himself to publish in an incomplete form. 

 Many of the papers which have been published by Prof. Stokes are 

 of so rigidly mathematical a character that their titles would fail to 

 convey any idea to the non-mathematical mind. To this category be- 

 long the papers entitled "Critical Values of the Sums of Periodic 

 Changes " and " Numerical Calculation of Definite Integrals and Infi- 

 nite Series." The following incomplete list will serve to show the 

 comprehensiveness of Prof. Stokes's researches in applied mathe- 

 matics : 



"On the Friction of Fluids in Motion, and the Eqitilibrium and 

 Motion of Elastic Solids," 1845 ; " Effects of the Internal Friction of 

 Fluids on the Motion of Pendulums," 1850. 



Of Stokes's papers stating the results of his researches on the 

 " Undulatory Theory of Light," three are cited by Prof. Tait, viz. : 

 "Dynamical Theory of Diffraction," 1849; "On the Colors of Thick 

 Plates," 1851; "On the Formation of the Central Spot of Newton's 

 Rings beyond the Critical Angle," 1848. 



The " Report on Double Refraction," in the " British Associa- 

 tion Reports for 1862," was drawn up by Prof. Stokes. 



"On the Vai'iation of Gravity at the Surface of the Earth," 1849. 



"On the Change of the Refrangibility of Light," 1852. This 

 paper contains his famous experimental explanation of fluorescence, 

 which earned for its author his fellowship in the Royal Society. 



Among the papers published by Stokes since the year 1864, two 

 are specially worthy of mention, viz. : " On the Long Spectrum of 

 Electric Light," and " On the Absorption Spectrum of Blood." 



In conjunction with the late Mr. Vernon Harcourt, Stokes made a 

 highly-valuable experimental inquiry into what is called Irrational- 

 ity of Dispersion, chiefly with a view to the improvement of achro- 

 matic telescopes. 



" There can be no doubt," writes Prof. Tait, " as was well shown 

 by Sir W. Thomson in his presidential Address to the British Associ- 



