where progress demands that these distinct and rarely associated powers be brought simultaneously into action. For there the mathematician 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 learned; 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 mathematical 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 published 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 belong the papers entitled "Critical Values of the Sums of Periodic Changes" and "Numerical Calculation of Definite Integrals and Infinite Series." The following incomplete list will serve to show the comprehensiveness of Prof. Stokes's researches in applied mathematics:
"On the Friction of Fluids in Motion, and the Equilibrium 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 Association Reports for 1862," was drawn up by Prof. Stokes.
"On the Variation 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 Irrationality of Dispersion, chiefly with a view to the improvement of achromatic 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-
