246 



NATURE 



[July ii, 1895 



represent the I'niversity of London in Parliament. I 

 find, however, that your letter addressed to me is imder- 

 stood to show that 1 ajjrec with all the opinions expressed 

 by you in that letter, and in justice to myself I must make 

 known to my fellow electors and others how wholly we 

 disagree on the above point, and how much 1 regret the 

 attitude you assume in the matters in question. 



".\'ours \er>- truly, 

 " Sir John Lubbock, Bart.*" " M. Foster. 



The second has been addressed to Sir John Lubbock 

 by the President and a number of Fellows of the Royal 

 Society :— 



"July 6, 189;. 



" Dear .Sir John Lubbock,- The interests of learning 

 and of education are so closely bound up with the future 

 development of the I'niversity of London that we hope 

 you will not regard us as interfering between yourself and 

 the Electing Body of the University if we venture to ex- 

 press our regret at some of the opinions you have put 

 forvvard in your Election address. 



" You state that you would do your best to secure that 

 the scheme (for the reorganisation of the University), 

 when arranged, should be submitted to Convocation for 

 their approval, to be signified as at a .Senatorial Election, 

 and would oppose the Hill unless this were conceded. 



" You must allow us to point out that this proposal 

 would confer upon Convocation a right, which is without 

 precedent, to supervise the acts of a Commission en- 

 trusted with the reorganisation of the University of which 

 Convocation itself is a part. 



"The scheme of the ' Gresham Commissioners' has 

 been approved not only by all the institutions concerned, 

 but by the great body of educated public opinion. It is, 

 however, certain that very grave difficulties will arise if 

 the ultimate fate of the scheme is to depend upon the 

 votinu papers of Convocation. 



" We, therefore, believe that the proposal you support, 

 if adopted, will result in the failure of another attempt to 

 establish a Teaching L'niversity in London, and will in- 

 definitely postpone the solution of a question which, 

 after prolonged discussion, seemed to be on the eve of 

 settlement. 



" We are, yours faithfully, 

 " KKr.viN (P.R.S.), John Ev.w.s (Treas.R.S.), M. 

 Foster (Sec.R.S.), Joseph Lister, R.\vi,eu;h, 

 DorcL.vs G.VLTON, T. C. Bonnev, T. E. Thori-e, 



HOR.XCE L.\Mli, J. H. I'OVNTl.NT.. ARTHIR \V. 

 RtcKER. E. FR.\NKI..\NI), N. Storv M.xskelvxe, 

 Henry E. Roscoe, P. H. Pve-Smith, J. Normax 

 l.wKVER, John Erk- ERitHsEv, \Vii mam 

 Ramsay, G. Carev Foster." 



In his address. Sir John Lubbock states that the opinions 

 of the present Government on the I'niversity question 

 have yet to be made known. In view of the fact that the 

 Commission, whose report has been so generally ap- 

 proved, was appointed during Lord Salisbury's last term 

 of office, this attitude ought not to be doubtful. 



THE KI.ECTKICAL AfEASCKEMEW OF 

 ST AH LIGHT. 



IT'WXK the light of a star is able to produce al the 

 ■*■ •surface of the earth a measurable effect, other than 

 the anion on a photographic plate, is a fact which was 

 published in these pages in January last year. The light 

 of stars and planets produces two efTccts— the one photo- 

 graphic and the other electric. The first— which has, of 

 course, been known for many years is slow in its opera- 

 tion ; the second which was discovered only a year ago 

 in Mr. Wilson's obscrvator>' at Oaramona, Wcsimeath — 

 is almost instantaneous. 



NO. I 34 I. VOL. 52] 



In order to obtain the electrical effect, a photoelectric 

 cell of extremely great sensitiveness to light is employed. 

 Such a cell is constructed with selenium, aluminium. 

 and the liquid ctnanthol. If we take a strip of clean 

 aluminium -say half an inch long, one-tenth of an inch 

 wide, and thick enough to be fairly stift" lay it on an 

 iron i)Iatc which is heated by a Bunsen tlame, and place 

 on the end of the strip a veiy small particle of selenium, 

 this selenium will mell and form a small black globule 

 of liquid. Let the flame be now withdrawn, and the 

 globule of melted selenium spread over the end of the 

 aluminium strip, by means of a hot glass rod, so that il 

 forms a thin uniform layer of area about 'i of an inch 

 square on the end of the strip, and let this dark layer 

 cool to a few degrees below its melting point (about 217' 

 C). Now apply heat again to the under surface of the 

 iron plate until the aluminium strip becomes nearly hot 

 enough to re-melt the layer of selenium. In ihis process 

 the colour of the layer will gradually change from black 

 to a greyish brown. When it is just on the point of melt- 

 ing, withdraw the heat and blow over its surface ; this 

 will instantly check the tendency to melt, and will leave 

 the surface of the selenium in the state in which it is 

 most sensitive to light. If this strip (or rather its sele- 

 nium-covered end I is immersed in a glass tube contain- 

 ing acetone or ccnanthol, and connected with one pole of 

 a quadrant electrometer, whose other pole is connected 

 with a platinum wire scaled into the glass tul)e, we ha\ e 

 a photoelectric cell, in which the action of light falling 

 on the selenium layer results in giving the selenium a 

 positive electric charge and tlic liquid a negative one, the 

 former charge being conveyed to one pole of the electro- 

 meter by the aluminium plate, and the latter to the other 

 pole by the platinum wire scaled into the cell. 



Roughly speaking, the difference of potential produced 

 in such a cell as this by ordinary ditTused daylight 

 is something between one-third and one-half of a volt. 



Such were the seleno-aluminium cells used in the 

 measurement of starlight in January 1894, the liquid 

 in them being tenanthol. This liquid was found to be 

 better than acetone (which had been previously used), 

 not only because of the greater ease with which it can be 

 sealed up in glass tubes, but because it does not act 

 chemically on selenium, which acetone seems to do 

 sooner or later. But it is obvious that a cell formed in 

 this way contains an element of inconstancy ; for, the 

 strip of aUiminiiun will at the same time convey to the 

 insul.itcd |)nle of the electrometer the positive charge 

 generated by light in the selenium and a portion of the 

 negative charge imparted to the liquid, so that the 

 effective E. M.F". is less than it sliould be : and, again, 

 there will be currents circulating perpetually between the 

 selenium and the back of the ahuninium slii]), and such 

 currents deteriorate the cell. Hence it happened that 

 such cells always fell oft' in strength after about six hours. 

 They sufficed, however, to show very easily measurable 

 electromotive forces from the light of the planets, and 

 even from the light of Sirius. 



.Shortly after January 1S94, a very notable improve- 

 ment was made in the construction of the cells, this 

 improvement resulting from the perception of the cause 

 of deterioration above explained. Instead of a strip of 

 aluminium as a l)ase for the selenium layer, the end of 

 an aluminium wire, about one millimetre in diameter, 

 was used. This wire was enclosed in .1 glass tube iA, It, 

 in the figure on p. 247). into which it fitted tightly, one 

 end of the wire being flush with ;in end of the tube. 

 On this end was deposited the layer of selenium, with 

 the same process of heating as that already described. 

 The other end of the aluminium wire inside the glass 

 tube was connected with a fine platinum wire, I', which 

 emerged from the second enti of the tube, and which 

 formed the selenium pole of the photoelectric cell. 



In this way the liquid is kept out of contact with the 



