NATURE 



[June 23, 1898 



SOCIETIES AND ACADEMIES. 



London. 



Royal Society, May 26. — " Contributions to the study of 

 'Flicker.'" By T. C. Porter (Eton College). Communicated 

 by LordRayleigh, F.R.S. 



The first part of the paper describes experiments made to 

 ascertain the exact relative rotations at which the flicker of a 

 disc, half black, half coloured, vanishes in the different colours 

 of the spectra of sun- and lime light, formed by a diffraction 

 grating of 14,434 lines to the inch. The main precautions 

 which must be taken are briefly stated, with a short discussion 

 of the results, which may be summed up as follows. 



The rate of rotation of the disc that the flicker may just 

 vanish is highest for the yellow, decreasing for the succession 

 of colours on either side of this one, being the same for the 

 deepest visible crimson and full green ; from the full green to 

 the violet end of the spectrum the rate continues to fall off, till 

 in the last visible rays it is very nearly one-half its maximum 

 for the yellow. 



When the intensity of the different spectra is varied, the 

 greater the intensity the more rapid is the rate of rotation 

 necessary for flicker just to vanish ; thus, as the stimulus applied 

 to the retina increases in intensity, the impression produced 

 retains its maximum value for a shorter and shorter time. 

 That a brighter illumination of the disc does produce a greater 

 stimulus (/.e. that neither the contraction of the pupil, nor any 

 other cause, overcomes the effect of brighter illumination) is 

 proved by the fact that the brighter the light, the brighter on 

 the whole is the disc, when flicker has just vanished. Research 

 was made to discover in what way the rotation of a black and 

 coloured disc must be varied for flicker to vanish, when the 

 proportion of the coloured to the black sector varied by stages 

 of 10° at a time, the experiments being carried out in each of 

 the main colours of the lime-light spectrum. 



Throughout this series of experiments the intensity of the 

 illuminant was kept constant. The results are expressed in a 

 series of rather remarkable curves, the rate of rotation rising 

 rapidly with the instalments of 10° to the coloured sector, then 

 remaining at its maximum, and constant within the errors of 

 experiment, from a coloured sector of about 150° to one of 

 about 240°, after which the rate of rotation falls off somewhat 

 more rapidly than it rose when the coloured sector was small. 



The remainder of the paper is devoted to the discussion of 

 these curves, and it concludes by proving from a series of points, 

 taken at random on one of them, that the duration of the im- 

 pression on the retina undiminished is inversely proportional to 

 the time during which the retina is stimulated. Some other 

 conclusions of interest are arrived at ; but for the reasoning and 

 description of experiments necessary, we must refer to the paper 

 itself: e.g. (i) When flicker has just vanished, the effective 

 stimulus at any point of the retina is to the maximum 

 stimulus the coloured sector can produce, as the angle of the 

 yellow sector is to the angle of the whole disc {i.e. 360°), the 

 illumination being supposed constant. (2) The coloured sector 

 always requires a finite time in order to produce its maximum 

 effect on the retina. (3) When the width of the white or 

 coloured sector is increased in steps of 10" at a time, the 

 increment in the apparent brightness in the rotating and 

 flickerless disc follows, within the errors of experiment, the 

 series of i/o, i/i, 1/2, 1/3, 1/4, &c., as it should. The paper 

 is the first of a series on the subject. 



"Aluminium as an Electrode in Cells for Direct and Alter- 

 nate Currents." By Ernest Wilson. Communicated by Dr. J. 

 Hopkinson, F.R.S. 



This paper deals with the apparent great resistance which 

 aluminium offers to the passage of an electric current when 

 coated with a film and used as an anode in cells containing, for 

 instance, such an electrolyte as alum in water. 



Part i. deals with experiments made on aluminium-carbon 

 cells with direct currents, the electrolytes used being alum 

 solution, dilute sulphuric acid, and sodium hydrate in water. 



After making a preliminary experiment in which an exploring 

 electrode inserted between the plates was used to allocate the 

 distribution of potential difference in the cell, the author 

 describes a series of experiments made with a view to finding 

 the effect of variation of current density and temperature upon 

 the potential difference between the Al and C plates. Two cells 

 were constructed and, during the forming process, which con- 

 sisted of passing a current of -005 ampere per square inch of 



NO. 1495, VOL. 58] 



the Al anode for forty-seven hours, one of them contained a 

 dilute HjS04, and the other a saturated potash alum solution. 

 The apparent resistance was not nearly so marked in the H.^SOj 

 solution cell as in the other. 



The H2SO4 solution was then replaced by a saturated alum 

 solution, and the two cells submitted to a further forming pro- 

 cess for thirteen hours. The two cells were then experimented 

 upon at approximately constant temperature, but the current 

 density varied. The potential difference in the two cases rose 

 from 1-89 to 34-5 volts as the current density varied from 

 ■00006 to '12 in the case of the H.2S04-formed plate, and 

 from -oooii to •041 in the other. With about 39 volts directly 

 applied to each the current rapidly increased in the two cells, 

 accompanied by rapid increase of temperature from its final 

 value of about 20° C. • ., 



The cell containing the alum-formed plate was next heated .; 

 the current density of the Al anode kept approximately con- 

 stant at '019 ampere per square inch, and the temperature 

 raised from 13° to 70° C. The potential difference fell from 30 

 to 3 volts under these conditions. 



The potential difference was not materially increased by 

 cooling a cell in a freezing mixture of carbonic acid snow and 

 ether. 



One of these films was examined under a microscope and 

 analysed by Mr. Herbert Jackson, of King's College, London, 

 who states " that the skin over the plate is seen to be full of 

 minute cracks, giving the impression of a dried gelatinous 

 pellicle ; not an unexpected appearance if the plate had been 

 covered when wet with a thin coating of the gelatinous 

 aluminium hydroxide. The analysis of the film over the metal 

 shows it to consist of basic aluminium sulphate." 



An experiment was made upon a film which was formed 

 without the passage of current by first submerging a bright 

 Al plate in alum solution, and then exposing it to the 

 atmosphere. The author concludes that this film has the same 

 effect as another formed during the passage of current. 



Part ii. deals with alternate currents. Experimenting first 

 with AlC plates in potash alum solution, the frequency was 

 varied from 16 to 98 periods per second, and the current 

 density varied from '0396 to -583 ampere per square inch 

 of the Al plate. The results show that small currents 

 are accompanied by large phase difference ; , but the effect 

 looked for, namely a large ratio between the maximum 

 coulombs in the two halves of a period, has not time to properly 

 develop at the frequencies tried. 



The next set of experiments deals with a "rectifier" for 

 alternate currents proposed by Graetz, in which Al-C cells are 

 employed. It is shown that a uni-directional current can be 

 produced by the Graetz arrangement, and the efliciency of such 

 a system is discussed. 



The concluding portion of these experiments deals with 

 aluminium plates only for the purpose of forming the electrodes 

 of a condenser for alternate currents. Soda, ammonia, and 

 potash alums, both saturated and non-saturated, are employed 

 as electrolytes. The frequency is varied from 75 to 100 periods 

 per second, and the current density from "0139 to "6. The 

 results show that phase differences of the order of 60° and 70' 

 (360° = I period) can be obtained by a suitable choice of current 

 density and temperature. Maximum phase difference develops 

 with small currents at low temperature. With regard to 

 frequency, both saturated and non-saturated solutions give a 

 higher efficiency at the higher frequency. The conclusion is 

 that the metal aluminium is suitable for use a.s the plates of 

 condensers if due regard be given to current density and 

 temperature. 



Royal Microscopical Society, May 18. — Mr. E. M. 

 Nelson, President, in the chair. — The President exhibited a 

 simple form of student's microtome, suitable for cutting soft 

 sections. It was made by Messrs. Reynolds and Branson, of 

 Leeds, and was on the principle of the Williams microtome, 

 but consisted only of a plate of glass and an adjustable casting 

 carrying the razor. The chief point of interest was its low cost. 

 — The President read three short papers which had been re- 

 ceived from Mr. Jourdain. The first was on a new apochro- 

 matic objective constructed without the use of fluorite. This 

 objective is made by the Bausch and Lomb Optical Company, 

 who have promised to send examples to Mr. Jourdain for examin- 

 ation. The other papers by Mr. Jourdain were on a method 

 of adjusting the sizes of coloured images yielded by the Cooke 

 lens, and on the construction of the planar lens and its use in 



