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NATURE 



\_Feb. 5, 1880 



thoroughly characteristic of the man, that his mind could 

 never bear to pass by any phenomenon without satisfying 

 itself of at least its general nature and causes. 



In the same volume of the Math. Journal there is an 

 exceedingly elegant "problem" due to Maxwell, with his 

 solution of it. In a note we are told that it was " sug- 

 gested by the contemplation of the structure of the crys- 

 talline lens in fish." It is as follows : — 



A transparent medium is such that the path of a ray of 

 light within it is a given circle, the index of refraction 

 being a function of the distance from a given point in the 

 plane of the circle. Find the form of this function, and 

 show that for light of the same refrangibility — 



1. The path of every ray within the medium is a circle. 



2. All the rays proceeding from any point in the medium 

 will meet accurately in another point. 



3. If rays diverge from a point without the medium and 

 enter it through a spherical surface having that point for 

 its centre, they will be made to converge accurately to a 

 point within the medium. 



Analytical treatment of this and connected questions, 

 by a novel method, will be found in a paper by the present 

 writer {Trans. R.S.E. 1865). 



Optics was one of Clerk- Maxwell's favourite subjects, 

 but of his many papers on various branches of it, or 

 subjects directly connected with it, we need mention only 

 the following : — 



"On the General Laws of Optical Instruments" 

 (Quart. Math. Jour. 1858). 



" On the Cyclide " (Quart. Math. Journal, 1868). 



" On the best Arrangement for Producing a Pure 

 Spectrum on a Screen" (Proc. R.S.E. 1868). 



"On the Focal Lines of a Refracted Pencil" (Math. 

 Soc. Proc. 1873). 



A remarkable paper, for which he obtained the Keith 

 Prize of the Royal Society of Edinburgh, is entitled 

 " On Reciprocal Figures, Frames, and Diagrams of 

 Forces." It is published in the Transactions of the 

 Society for 1870. Portions of it had previously appeared 

 in the Phil. Mag. (1864). 



The triangle and the polygon of forces, as well as 

 the funicular polygon, had long been known ; and also 

 some corresponding elementary theorems connected with 

 hydrostatic pressure on the faces of a polyhedron : but 

 it is to Rankine that we owe the full principle of dia- 

 grams, and reciprocal diagrams, of frames and of forces. 

 Maxwell has greatly simplified and extended Rankine' s 

 ideas : on the one hand facilitating their application 

 to practical problems of construction, and on the other 

 hand extending the principle to the general subject of 

 stress in bodies. The paper concludes with a valuable 

 extension to three dimensions of Sir George Airy's 

 " Function of Stress." 



His contributions to the Proceedings of the London 

 Mathematical Society were numerous and valuable. I 

 select as a typical specimen his paper on the forms of 

 the stream-lines when a circular cylinder is moved in a 

 straight line, perpendicular to its axis, through an infi- 

 nitely extended, frictionless, incompressible fluid (vol. 

 iii. p. 224). He gives the complete solution of the 

 problem ; and, with his usual graphical skill, so promi- 

 nent in his great work on Electricity, gives diagrams of 

 the stream-lines, and of the paths of individual particles 



of the fluid. The results are both interesting and in- 

 structive in the highest degree. 



In addition to those we have mentioned we cannot 

 recall many pieces of experimental work on Maxwell's 

 part: — with two grand exceptions. The first was con- 

 nected with the determination of the British Association 

 Unit of Electric Resistance, and the closely associated 

 measurement of the ratio of the electrokinetic to the 

 electrostatic unit. In this he was associated with Pro- 

 fessors Balfour Stewart and Jenkin. The Reports of that 

 Committee are among the most valuable physical papers 

 of the age ; and are now obtainable in a book-form, 

 separately published. The second was the experimental 

 verification of Ohm' s law to an exceedingly close approxi- 

 mation, which was made by him at the Cavendish Labo- 

 ratory with the assistance of Prof. ChrystaL. 



In his undergraduate days he made an experiment 

 which, though to a certain extent physiological, was 

 closely connected with physics. Its object was to deter- 

 mine why a cat always lights on its feet, however it may 

 be let fall. He satisfied himself, by pitching a cat gently 

 on a mattress stretched on the floor, giving it different 

 initial amounts of rotation, that it instinctively made use 

 of the conservation of Moment of Momentum, by stretching 

 out its body if it were rotating so fast as otherwise to fall 

 head foremost, and by drawing itself together if it were 

 rotating too slowly. 



I have given in this journal (vol. xvi. p. 119) a de- 

 tailed account of his remarkable elementary treatise 

 on "Matter and Motion," a work full of most valuable 

 materials, and worthy of most attentive perusal not merely 

 by students but by the foremost of scientific men. 



His "Theory of Heat," which has already gone through 

 several editions, is professedly elementary, but in many 

 places is probably, in spite of its admirable definiteness, 

 more difficult to follow than any other of his writings. 

 In intrinsic importance it is of the same high order as his 

 " Electricity," but as a whole it is not an elementary 

 book. One of the few knowable things which Clerk- 

 Maxwell did not know, was the distinction which most 

 men readily perceive between what is easy and what is 

 hard. What he called hard, others would be inclined to call 

 altogether unintelligible. In thelittlebookwearediscussing 

 there is matter enough to fill two or three large volumes 

 without undue dilution (perhaps we should rather say, 

 with the necessary dilution) of its varied contents. There 

 is nothing flabby, so to speak, about anything Maxwell 

 ever wrote : there is splendid muscle throughout, and an 

 adequate bony structure to support it. " Strong meat for 

 grown men " was one of his favourite expressions of com- 

 mendation ; and no man ever more happily exposed the true 

 nature of the so-called "popular science" of modern 

 times than he did when he wrote of " the forcible language 

 " and striking illustrations by which those who are past hope 

 " of being even beginners [in science] are prevented from 

 " becoming conscious of intellectual exhaustion before 

 " the hour has elapsed." 



To the long list of works attached to Maxwell's name 

 in the Royal Society's Catalogue of Scientific Papers may 

 now be added his numerous contributions to the latest 

 edition of the " Encyclopedia Britannica"— Atom, At- 

 traction, Capillarity, &c. Also the laborious task of pre- 

 paring for the press, with copious and very valuable 



