August 2%, 1884] 



NATURE 



4i5 



The instrumental weapon of investigation, the spectroscope 

 itself, has made important advances. On the theoretical side, 

 we have for our guidance the law that the optical power in 

 gratings is proportional to the total number of lines accurately 

 ruled, without regard to the degree of closeness, and in prisms that 

 it is proportional to the thickness of glass traversed. The mag- 

 nificent gratings of Rowland are a new power in the hands of 

 the spectroscopist, and as triumphs of mechanical art seem to be 

 little short of perfection. In our own report for 18S2 Mr. 

 Mallock has described a machine, constructed by him, for ruling 

 large diffraction gratings, similar in some respects to that of 

 Rowland. 



The great optical constant, the velocity of light, has been the 

 subject of three distinct investigations by Cornu, Michelson, and 

 Forbes. As may be supposed, the matter is of no ordinary 

 difficulty, and it is therefore not surprising that the agreement 

 should be less decided than could be wished. From their obser- 

 vations, which were made by a modification of Fizeau's method 

 of the toothed wheel, Young and Forbes drew the conclusion 

 that the velocity of light in vacuo varies from colour to colour, 

 to such an extent that the velocity of blue light is nearly 2 per 

 cent, greater than that of red light. Such a variation is quite 

 opposed to existing theoretical notions, and could only be 

 accepted on the strongest evidence. Mr. Michelson, whose 

 method (that of Foucault) is well suited to bring into prominence 

 a variation of velocity with wave-length, informs me that he has 

 recently repeated his experiments with special reference to the 

 point in question, and has arrived at the conclusion that no 

 variation exists comparable with that asserted by Young and 

 Forbes. The actual velocity differs little from that found from 

 his first series of experiments, and may be taken to be 299,800 

 km. per second. 



It is remarkable how many of the playthings of our childhood 

 give rise to questions of the deepest scientific interest. The top 

 is, or may be, understood, but a complete comprehension of the 

 kite and of the soap-bubble would carry us far beyond our present 

 stage of knowledge. In spite of the admirable investigations of 

 Plateau, it still remains a mystery why soapy water stands almost 

 alone among fluids as a material for bubbles. The beautiful 

 development of colour was long ago ascribed to the interference 

 of light, called into play by the gradual thinning of the film. In 

 accordance with this view the tint is determined solely by the 

 thickness of the film, and the refractive index of the fluid. Some 

 of the phenomena are, however, so curious as to have led 

 excellent observers like Brewster to reject the theory of thin 

 plates, and to assume the secretion of various kinds of colouring 

 matter. If the rim of a wine-glass be dipped in soapy water, 

 and then held in a vertical position, horizontal bands soon begin 

 to show at the top of the film, and extend themselves gradually 

 downwards. According to Brewster these bands are not formed 

 by the "subsidence and gradual thinning of the film," because 

 they maintain their horizontal position when the glass is turned 

 round its axis. The experiment is both easy and interesting ; but 

 the conclusion drawn from it cannot be accepted. The fact is 

 that the various parts oftheMilm cannot quickly alter their thick - 

 ness, and hence when the glass is rotated they rearrange them- 

 selves in order of superficial density, the thinner parts floating up 

 over, or through, the thicker parts. Only thus can the tendency 

 be satisfied for the centre of gravity to assume the lowest possible 

 position. 



When the thickness of a film falls below a small fraction of 

 the length of a wave of light, the colour disappears and is re- 

 placed by an intense blackness. Profs. Reinold and Riicker 

 have recently made the remarkable observation that the whole of 

 the black region, soon after its formation, is of uniform thick- 

 ness, the 1 assage from the black to the coloured portions being 

 exceedingly abrupt. By two independent methods they have 

 determined the thickness of the black film to lie between seven 

 and fourteen millionths of a millimetre ; so that the thinnest 

 films correspond to about one-seventieth of a wave-length of 

 light. The importance of these results in regard to molecular 

 theory is too obvious to be insisted upon. 



The beautiful inventions of the telephone and the phonograph, 

 although in the main dependent upon principles long since 

 established, have imparted a new interest to the study of 

 acoustics. The former, apart from its uses in every-day life, 

 has become in the hands of its inventor, Graham Bell, and of 

 Hughes, an instrument of first-class scientific importance. The 

 theory of its action is still in some respects obscure, as is shown 



by the comparative failure of the many attempts to improve it. 

 In connection with some explanations that have been offered, we 

 do well to remember that molecular changes in solid masses are 

 inaudible in themselves, and can only be manifested to our ears 

 by the generation of a to-and-fro motion of the external surface 

 extending over a sensible area. If the surface of a solid remains 

 undisturbed, our ears can tell us nothing of what goes on in the 

 interior. 



In theoretical acoustics progress has been steadily maintained,, 

 and many phenomena which were obscure twenty or thirty years 

 ago, have since received adequate explanation. If some im- 

 portant practical questions remain unsolved, one reason is that 

 they have not yet been definitely stated. Almost everything in 

 connection with the ordinary use of our senses presents peculiar 

 difficulties to scientific investigation. Some kinds of information 

 with regard to their surroundings are of such paramount import- 

 ance to successive generations of living beings, that they have 

 learned to interpret indications which, from a physical point of 

 view, are of the slenderest character. Every day we are in the 

 habit of recognising, without much difficulty, the quarter from 

 which a sound proceeds, but by what steps we attain that end 

 has not yet been satisfactorily explained. It has been proved 

 that when proper precautions are taken we are unable to dis- 

 tinguish whether a pure tone (as from a vibrating tuning-fork 

 held over a suitable resonator) comes to us from in front or from 

 behind. This is what might have been expected from an a 

 priori point of view ; but what would not have been expected is 

 that with almost any other sort of sound, from a clap of the 

 hands to the clearest vowel sound, the discrimination is not only 

 possible, but easy and instinctive. In these cases it does not 

 appear how the possession of two ears helps us, though there is 

 some evidence that it does ; and even when sounds come to us 

 from the right or left, the explanation of the ready discrimina- 

 tion which is then possible with pure tones is not so easy as 

 might at first appear. We should be inclined to think that the 

 sound was heard much more loudly with the ear that is turned 

 towards than with the ear that is turned from it, and that in this 

 way the direction was recognised. But if we try the experiment 

 we find that, at any rate with notes near the middle of the 

 musical scale, the difference of loudness is by no means so very 

 great. The wave-lengths of such notes are long enough in rela- 

 tion to the dimensions of the head to forbid the formation of 

 anything like a sound shadow in which the averted ear might be 

 sheltered. 



In concluding this imperfect survey of recent progress in 

 physics, I must warn you emphatiially that much of great im- 

 portance has been passed over altogether. I should have liked 

 to speak to you of those far-reaching speculations, especially 

 associated with the name of Maxwell, in which light is regarded 

 as a disturbance in an electro-magnetic medium. Indeed, at one 

 time I had thought of taking the scientific work of Maxwell as 

 the principal theme of this address. But, like most men of 

 genius, Maxwell delighted in questions too obscure and difficult 

 for hasty treatment, and thus much of his work could hardly be 

 considered upon such an occasion as the present. His biography 

 has recently been published, and should be read by all who are 

 interested in science and in scientific men. His many-sided 

 character, the quaintness of his humour, the penetration of his 

 intellect, his simple but deep religious feeling, the affection 

 between son and father, the devotion of husband to wife, all 

 combine to form a rare and fascinating picture. To estimate 

 rightly his influence upon the present state of science, we must 

 regard not only the work that he executed himself, important as 

 that was, but also the ideas and the spirit which he communi- 

 cated to others. Speaking for myself as one who in a special 

 sense entered into his labours, I should find it difficult to express 

 adequately my feeling of obligation. The impress of his thoughts 

 may be recognised in much of the best work of the present time. 

 As a teacher and examiner he was well acquainted with the 

 almost universal tendency of uninstructed minds to elevate 

 phrases above things : to refer, for example, to the principle of 

 the conservation of energy for an explanation of the persistent 

 rotation of a fly-wheel, almost in the style of the doctor in " Le 

 Malade Imaginaire," who explains the fact that opium sends you 

 to sleep by its soporific virtue. Maxwell's endeavour was 

 always to keep the facts in the foreground, and to his influence, 

 in conjunction with that of Thomson and Helmholtz, is largely 

 due that elimination of unnecessary hypothesis which is one of 

 the distinguishing characteristics of the science of the present day. 



