June 14, 1894] 



NA TURE 



161 



clean surface of metals under the action of light is really a 

 chemical phenomenon. It had been asserted by some experi- 

 menters that the most oxidisable metals acted most powerfully, 

 but my own experience renders this doubtful ; I now find thaf 

 gold platinum and carbon discharge with very fair rapidity, and 

 that nearly all substances have some discharging power. A 

 few materials, cobalt among metals, discharge positive electrifi- 

 cation more rapidly than negative. The whole matter is there- 

 fore now under investigation. 



In the footnote to same column, end of first paragraph, the 

 word "even" should be deleted. The assertion intended is 

 that dried soil discharges rapidly, while damp soil discharges 

 only slowly. 



Same page, middle of second column, "two years ago" 

 should be /oa'-jft-arj a^w ; !-ince Fitzgerald's Royal Institution 

 Lecture was delivered in March 1890, and reported in Nature 

 of June 19 the same year. 



Lower down, the name Kolacec, preceding that of D. E. 

 [ones, has been omitted. 



Page 13S, second column, with reference to the reflecting 

 power of different substances it may be interesting to give the 

 following numbers, showing the motion of the spot of light 

 when S-inch waves were reflected into the copper hat, the 



angle of incidence being about 45°, by the following 

 mirrors : — 



Sheet of window glass 



Human body 



Drawing board 



Towel soaked with tap-water 



Tea-paper (lead?)... 



Dutcti metal paper 



Tinfoil 



Sheet copper 



or at most 



division. 



divisions. 



7 

 12 



12 ,, 



40 

 70 

 go 

 100 and up against 

 stops. 

 Page 139. It would have been clearer if the penultimate 

 paragraph, beginning " To demonstrate," had run thus : — 



To demonstrate ttiat the so-called plane of polarisation of 

 the radiation transmitted by a grid is at right angles to the 

 electric vibration, i.e. that when light is reflected from the 

 boundary of a transparent substance at the polarising angle the 

 electric vibrations of the reflected beam are perpendicular to 

 the plane of reflection, I use, S:c. 



The following is a copy of one of the wall-diagrams ; it is 

 interesting as showing how numerous the now-known detectors 

 of radiation are : — 



DETECTORS OF RADIATION. 



the frog's leg indirates that it does not appear really to respond to radiation, unless stimulated in some secondary manser. 

 er things are unimportant, but suggest the persons who applied the detector to eltciric radiation. The query against Selenium is 



X The cross against _ _ 

 The names against the other thmgs are unimportant, but suggest the persons 

 placed there because of uncertainty as to its most appropriate column. 



STUDY OF FLUID MOTION BY MEANS OF 

 COLOURED BANDS. ^ 



TN his charming story of "The Purloined Letter," Edgar 

 Allan Poe tells how all the efforts and artifices of the Paris 

 police to obtain possession of a certain letter, known to be in a 

 particular room, were completely baffled for months by the 

 simple plan of leaving the letter in an unsealed envelope in a 

 letler-rack, and so destroying all curiosity as to its contents ; 

 and how the letter was at last found there by a young man who 

 was not a professional member of the force. Closely analogous to 

 this is the story I have to set before you tonight — how certain 

 mysteries of fluid motion, which have resisted all attempts to pene- 

 trate them, are at last explained by the simplest means and in 

 the most obvious manner. 



This indeed is no new story in science. The method adopted 

 liy the minister, D., to secrete his letter appears to be the 

 l.ivourite of nature in keeping her secrets, and the history of 

 -cience teems with instances in which keys, after being long 

 sought amongst the grander phenomena, have been found at 

 last not hidden with care, but scattered about, almost openly, 

 in the most commonplace incidents of everyday life which have 

 excited no curiosity. 



' A leclure delivered at the Royal Institution of Great Britain by Prof. 

 ' '-borne Reynolds. F.R.S. 



This was the case in physical astronomy — to which I shall 

 return after having reminded you that the motion of matter in 

 the universe naturally divides itself into three classes. 



(i) The motion of bodies as a whole — as a grand illustration 

 of which we have the heavenly bodies, or more humble, but 

 not less effective, the motion of a pendulum, or a falling body. 



(2) The relative motion of the different parts of the same 

 fluid or elastic body — for the illustration of which we may goto 

 the grand phenomena presented by the tide, the whirlwind, or 

 the transmission of sound, but which is equally well illustrated 

 by the oscillatory motion of the wave, as shown by the motion 

 of its surface and by the motion of this jelly, which, although 

 the most homely illustration, affords by far the best illustration 

 of the properties of an elastic solid. 



tj) The inter-motionsof a number of bodies amongst each other 

 — to which cl.ass belong the motions of the molecules of matter 

 resulting from heat, as the motions of the molecules of a gas, 

 in illustration of which I may mention the motions of indi- 

 viduals in a crowd, and illustrate by the motion of the grains 

 in this bottle when it is shaken, during which the white grains 

 at the top gradually mingle with the black ones at the bottom — 

 which inter-diffusion takes an important part in the method of 

 coloured bands. 



Now of these three classes of motion that of the individual 

 body is incomparably the simplest. Vet, as presented in the 



NO. 1285, VOL. 50] 



