September u. 1893] 



NA TV RE 



473 



SECTION A. 



MATHEMATICS AND PHYSICS. 



Opening Address by R. T. Glazebrook, M.A., F.R.S., 

 President of the Section. 



Before dealing with ihe subject which I hope to bring to 

 your notice this morning, I wish to express my deep regret for 

 the circumstances which have prevented Prof. Clifton, who had 

 accepted the nomination of the Council, from beirg your Presi- 

 dent this year. , r t- 



It was specially fitting that he who has done so iriuch for this 

 college, and particularly for this laboratory in which we meet, 

 should take the chair at Nottingham. The occasions on which 

 we see him are all too seldom ; and we who come frequently to 

 these meetings were looking forward to help and encourage- 

 ment in our work, derived from his wide experience. You 

 would desire, I feel sure, that I should convey to him the expres- 

 sions of your sympathy. For myself I must ask that you will 

 pass a lenient judgment on my efforts to fill his place. 



Let me commence, then, with a brief retrospect of the past 

 year and the events which concern our Section. 



From the days of Galileo the four satellites of Jupiter have 

 been ol jects i f interest to the astronomer. Their existence was 

 one of the earliest of the discoveries of the telescope ; they 

 proved conclusively that all the bodies of the solar system did 

 not move round the earth. The year which has passed since 

 our last meeting is memorable for the discovery of a fifth satellite. 

 It is a year today \September 13-14, 1892) since Prof. Bar- 

 nard convinced himself that he had seen with the great 

 telescope of the Lick Observatory this new member of our sys- 

 tem as a star of the thirteenth magnitude, revolving round the 

 planet in 1 1 hours 57 minutes 23 seconds.i 



The conference on electrical standard^, held at our meeting 

 last year, has had important results. The resolutions adopted 

 at Edinburgh were communicated to the Standards Committee 

 of the Board of Trade. A supplementary report accepting 

 these resolutions was agreed to by that Committee (November 

 29, 1892), and presented to the President of the Board of 

 Trade. The definitions contained in this report will be made the 

 basis of legislation throughout the world. They have been 

 accepted by France, Germany, Austria, and Italy. The con- 

 gress at Chicago, which has just been held, has ratified them, 

 and thus we may claim that your Committee, co operating with 

 the leaders of physical science in other lands, has secured inter- 

 national agreement on these fundamental points. 



Among the physical papers of the year I would mention a 

 few as specially calling for notice. Mr. E. H. Griffiths's re- 

 determination of the value of the mechanical equivalent of heat 

 has just been lublished {Phil. Trans, vol. clxxxiv. ), and is a 

 monumental work. With untiring energy and great ability he 

 struggled for five years against the difficulties of his task, and 

 has produced results which, with the exception of one group of 

 experiments, do not differ by more than I part in 10,000 ; while 

 the results of that one excepted group differ from the mean 

 only by i part in 4000. 



The number of ergs of work required to raise one gramme 

 of water 1" C. at 15° C. is 4'I98 x 10'. Expressed in foot- 

 pounds and Fahrenheit degrees, the value of J is 779'97- ^^^ 

 value obtained by Joule from his experiments on the friction of 

 water, when corrected in 1880 by Rowland so as to reduce his 

 readingsto the air thermometer, is 778'5 at 12° 7 C. The result 

 at this temperature of Rowland's own valuihle research is 7801. 

 Another satisfactory outcome of Mr. Griffiths's work is the very 

 exact accordance between the scale of temperature as determined 

 by the comparison of his platinum thermometer with the air 

 thermometer, which was made by Callendar and himself in 

 1890, and that of the nitrogen thermometer of the Bureau 

 International at Sevres. 



Another great work now happily complete is Rowland's 

 "Table of Standard Wave-lengths " (/%j/. jl/ir^., July, 1893). 

 Nearly a thousand lines have been measured with the skill and 

 accuracy for which Rowland has made himself famous ; and 

 in this table we see the results achieved by the genius which 

 lesigned the concave grating and the mechanical ingenuity 

 which contrived the almost perfect screw. 



Those of us who have seen Mr. Higgs's wonderful photo- 



''* In general," he says, " tfie s.itel!Ite has been faint On the 



13th, however, when the air was very clear, it was quite easy." — Naturk, 

 October 20, 1892. 



NO. 1246, VOL. 48] 



graphs of the solar spectrum, taken with a Rowland grating, 

 will rejoice to know that his map also is now finisheH. 



Lord Rayleigh's paper on " The Intensity of Light reflected 

 from Water and Mercury at nearly perpendicular incidence," 

 {Phil. .Mag., October, 1892), combined with the experiments on 

 reflexion from liquid surfaces in the neighbourhood of ihe 

 polarising angle {Phil. J/<r^., January, 1892), establishes results of 

 the utmost importance to optical theory. "There is thus," Lord 

 Rayleigh concludes, "no experimental evidence against the 

 rigorous application ofFresnel's formulae "—for the reflexion of 

 polarised light— " to the ideal case of an abrupt transition 

 between two uniform transparent media. ' 



Prof. Dawar has, during the year, continued his experiments 

 on the liquefaction of oxygen and nitrogen on a large scale. 

 To a physicist perhaps the most important n suits of the re- 

 search are the discovery of the magnetic properties of liquid 

 oxygen, and the proof of the fact that the resistance of certain 

 pure metals vanishes at absolute zero {Phil. Mag., October, 

 1892). The last discovery is borne out by Griffiths and 

 Callendar's experiments with their platinum thermometers 

 {Phil. Mag., December, 1892). 



Mr. \Villiam»'s article on "The Relation of the pimen- 

 sions of Physical Quantities to Directions in Space" (Phil. 

 Mag., September, 1892) has led to an interesting discussion. 

 Some of his deductions will be noticed later. 



The title-page of the first edition of Maxwell's " Electri- 

 city and Magnetism " bears the date 1873. This year, 1893, 

 we welcome a third edition, edited by Maxwell's distinguished 

 successor, and enriched by a supplementary volumf, in which 

 Prof. J. J. Thomson describes some of the advances made by 

 electrical science in the last twenty years. The subject matter 

 of this volume might well serve as a text for a Presidential 

 Address. 



The choice of a subject on which to speak to-day has been 

 no easy task. The field of physics and mathematics is a wide 

 one. There is one matter, however, to which for a few minutes 

 I should like to call your attention, inadequately though it be. 

 Optical theories have, since the year 1876, when I first read Sir 

 George Stokes's "Report on Double Refraction" (British 

 Association Report, 1862), had a special interest for me, and I 

 think the time ha^ come when we may with advantage review 

 our position with regard to them, and sum up our knowledge.' 



That light is propagated by an undulatory motion through 

 a medium which we call the ether is now an established fact, 

 although we know but little of the nature or constiiulion of the 

 ether. The history of this undulatory theory is full of interest, 

 and has, it appears to me, in its earlier stages been not quite 

 clearly apprehended. Two theories have lieen proposed to 

 account for optical phenomena. Descartes was the author of 

 the one, the emission theory. Hooke, though his work was 

 very incomplete, was the founder of the undulatory theory. In 

 his "Micrographia," 1664, page 56, he asserts tl1.1t liihtis a 

 quick and short vibratory motion, "propagated every way 

 through an homogeneous medium by direct or straight lines 

 extended every way like rays from the centre of a sphere. . . . 

 Every pulse or vibration of the luminous body will generate a 

 sphere which will continually increase and i;row bigger, just 

 after the same manner, though in lefiniiely swifter, as the waves 

 or rings on the surface do swell into bigger and bigger circles 

 about a point on it " ; and he gives on this hypothesis an account 

 of reflexion, refraction, dispersion, and the colours of thin 

 plates. In the same work, page 58, he describes an experi- 

 ment practically identical with Newton's famous prism experi- 

 ment, published in 1672. Ilooke used lor a prism a glass 

 vessel about two feet long, filled with water, and inclined so 

 that the sun's rays might enter obliquely at the upper surface 

 and traverse the water. "The top surface is covered by an 

 opacous body, all but a hole through which the sun's beams are 

 sufl"ered to pass into the water, and are thereby refracted " to 

 the b'lttom of the glass, "against which part if a paper be ex- 

 panded on the outside there will appear all the colours of the 

 rainbow— that is, there will be generated the two principal 

 colours, scarlet and blue, with all the' intermediate ories which 

 arise from the composiiion and diluting of these two." But 

 Hooke could make no use of his own observation ; he at- 

 tempted to substantiate from it his own theory of colours, and 



■ This address was in the printer's hands when I saw Sir George Stokes's 

 paper on " The Luininifcrous Ethw," Nature, July 27. H.tI I Itnown 

 ihat so great a master of my s ilijecl nad dealt with it so 'atelv, my choice 

 might have been different ; under the circumstances it was too late to change. 



