November i, 1900] 



NATURE 



23 



it. On ihe other hand, the Chief of the Weather Bureau would 

 at any time be glad to receive communications referring to the 

 physics of the atmosphere. — Dr. Shelford Bidwell then exhibited 

 some experiments illustrating phenomena of vision. The first 

 phenomenon illustrated was that known as " Recurrent Vision." 

 A vacuum tube, illuminated by an induction coil, was made to 

 rotate about a horizontal axis, and was seen to be followed, at an 

 angle of about forty degrees, by a feebly luminous reproduction 

 of itself. A spot of white light, projected upon a screen, and 

 caused to move slowly in a circular path, was also followed by a 

 less luminous spot. The same effect was shown by spots of 

 green and yellow light, but in the case of red light no ghost was 

 visible. The phenomena of recurrent vision are due principally, 

 if not entirely, to the action of violet nerve fibres. The next 

 experiments related to the non-achromatism of the eye. The 

 lenses of the eye do not constitute an achromatic combination, 

 although under ordinary conditions a bright object is not sur- 

 rounded by fringes of colour. The effects of chromatic aberra- 

 tion are disguised by the luminous haze which surrounds the 

 object, produced by a defect in the eye regarded as an optical 

 instrument. A six-rayed star, formed by cutting a hole in an 

 opaque screen, was illuminated by a gauze-covered condenser 

 containing an incandescent lamp. The star was fairly clearly 

 defined, and there were no fringes. More attentive observation 

 showed a luminous haze. This haze is formed in consequence 

 of the cellular structure of the eye, and the brightest rays — 

 orange, yellow and green — are chiefly instrumental in forming 

 it. If, therefore, these rays are obstructed, the conditions are 

 more favourable for the observation of chromatic aberration. 

 The rays were consequently cut ofT by means of coloured glasses, 

 and the general" hue of the star was purple ; to some it 

 appeared bordered with dark blue, while toothers (long-sighted) 

 it appeared bordered with red. Two oblong patches, one red 

 and the other blue-violet, and of approximately the same in- 

 tensity, were then produced side by side upon a screen. An 

 observer with very good eyesight was able, at a distance of ten 

 feet, to focus the patches alternately with perfect distinctness. 

 In general, the blue patch was said to be more or less blurred. 

 With an achromatic eye it should be possible to focus both 

 together. Dr. Bidwell then showed some lantern slides, illus- 

 trating the complex form seen when viewing a small luminous 

 spot through a gauze-covered lens placed so as not to be in 

 exact focus. Some experiments were performed illustrating the 

 principle of the colour top. When a bright image is formed 

 on the retina after a period of darkness it has, in general, a 

 red border which lasts for a fraction of a second. A dark 

 patch suddenly formed on a bright ground has a blue bor ier 

 which lasts for a similar time. These effects were attributed 

 by Dr. Bidwell to a sympathetic action of the red nerve fibres. 

 When the various nerve fibres occupying a limited portion of 

 the retina are stimulated by ordinary white or yellow light, the 

 immediately surrounding red nerve fibres are for a short period 

 excited sympathetically, while the violet or blue and green fibres 

 are not so excited, or in a much less degree. Again, when 

 light is suddenly cut off from a patch in a bright field, there 

 occurs a sympathetic insensitive reaction in the red fibres just 

 outside the darkened patch, in virtue of which they cease for a 

 moment to respond to the luminous stimulus ; the green and 

 violet fibres by continuing to respond uninterruptedly give rise 

 to the sensation of a blue border. By a simple experiment it 

 was shown that the explanation of the colour top, depending 

 upon changes in the convexity of the eye and non-achromatism, 

 was untenable. By the use of a strong light it is possible 

 to get negative after-images after looking at a brightly- 

 coloured object. These images are complementary in colour 

 to the object, and are formed even if the object is only 

 viewed for a fraction of a second. By means of proper illumin- 

 ation and a disc rotating at the proper speed, a red wafer was 

 so arranged that, upon looking at it, it was impossible to recog- 

 nise the wafer itself, but only the continuous green after-image. 

 The Chairman expressed his interest in the last experiment, in 

 which it was possible to see the negative after-image of an 

 object and not the object itself. Prof. S. P. Thompson said 

 these experiments threw a doubt on some of the accepted 

 notions about the properties of the eye. Dr. Bidwell asks us to 

 believe that the yellow haze is due to a cellular structure in the 

 eye. Is there such a structure ? Can it be observed with a micro- 

 scope ? And do its meshes correspond in magnitude with those 

 necessary to produce the effects ? By diminishing the size of 

 the pupil the haze is diminished and the sharpness of the image 



NO. 16 18, VOL. 63] 



is increased. Tne effects seem to ba due to ordinary aberration. 

 Prof. Thompson said that the achromatism. of the eye was 

 simply shown by covering half the object-glass of a telescope 

 and viewing a bright object with it. The object then seems 

 bordered with coloured fringes. Mr. Blakesley, referring to the 

 colour patches used by Dr. Bidwell, pointed out that although 

 the patches were the same distance from the lens, yet they did 

 not possess the same magnification. The last experiment shown 

 did away with the theory of persistence of vision, because the 

 space between the object and the negative after-image was 

 evidently not illuminated. Mr. Trotter asked if red and green 

 were the only colours which gave complementary negative 

 after-images. Dr. Bidwell, in reply, said the effect was 

 obtainable throughout the length of the spectrum. — A paper 

 on the concentration at the electrodes in a solution, 

 with special reference to the liberation of hydrogen by- 

 electrolysis of a mixture of copper sulphate and sulphuric 

 acid, was read by Dr. H. J. S. Sand. In this jjaper an equation 

 has been derived for calculating the concentration at the electrode 

 of a solution of a single salt from which the metal is being de- 

 posited under the conditions (t) that the solution is contained iiv 

 a cylindrical vessel bounded by the electrodes, (2) that no con- 

 vection-currents occur, and (3) that the diffusion of the salt obeys- 

 Fick's law, and its transport values are constant. This formula 

 can be made the basis for roughly determining diffusion co- 

 efficients. In the case of mixtures, it is possible to arrive at 

 limits for the concentration, and it has been experimentally- 

 proved that hydrogen always appears at the electrode of an acid 

 solution of copper sulphate, in which no currents of liquid are 

 taking place, between the limits of time for the concentration lo- 

 go down to zero. The time which it takes for the hydrc^en to 

 appear can be calculated from an empirical formula, which is 

 similar in form to the one used for a single salt. The great part 

 played by convection -currents in determining the ratio of the two> 

 constituents given off at the electrode of an acid copper-sulphate 

 solution, has been shown by proving experimentally that artificial 

 stirring causes hydrogen to disappear altogether in cases where 

 it would otherwise have presented over sixty per cent, of the 

 equivalents carrying the current from the solution to the elec-^ 

 trode. The Chairman drew attention to the fact that no hydrc^ei> 

 was liberated until all the copper had gone, and said the formula. 

 for the concentration might be used again in further investiga- 

 tions. Dr Donnan asked if the time at which hydrc^en was 

 liberated had been taken as the time at which hydrogen actually 

 made its appearance in the form of bubbles, or whether any 

 allowance had been made for saturation. Dr. Sand said the 

 time was taken up to the appearance of bubbles. — A paper by 

 Dr. R. A. Lehfeldt on electromotive force and osmotic pressure 

 was postponed until the next meeting. The meeting thea 

 adjourned until November 9. 



Paris. 

 Academy of Sciences, October 22. — M. Maurice Levy ir> 

 the chair. — On the convergence of meridians, by M. Hatt. — 

 Diagnosis of gaseous supersaturation in cases of a physical order 

 and chemical order, by M. Berthelot. A description is given of 

 attempts made to distinguish between these two classes of phe- 

 nomena by means of the calorimeter, the reactions studied being 

 the decomposition of dilute solutions of hydrogen peroxide by 

 addition of platinum black or of potassium permanganate. Fron> 

 experiments with the latter reagent, the conclusion is drawiv 

 that the considerable quantities of oxygen held in solution are 

 held in the state of an unstable chemical compound. — The origin 

 of atmospheric hydrogen, by M. Armand Gautier. It has been 

 shown in previous papers that air normally contains about 'oz 

 per cent, of free hydrogen. It has been shown that, besides being 

 a normal product of some putrefactive fermentations, hydrogen is 

 given off by many volcanoes, and also escapes from many mineral 

 springs. It is found that certain granites treated in vacuo with 

 phosphoric acid give about from three to four times their volume of 

 free hydrogen. Since ammonia is always produced at the same 

 time, the surmise is put forward that nitride of iron, FegNo, is the 

 source of these two gases. This nitride has not been isolated 

 from these granites, but iron nitrides have been found in the 

 crystalline deposits of the lava fissures of Etna by Silvestri.— 

 Observations on the development of the Onychophorje, by 

 M. E. L. Bouvier. The species, Peripatopsis Sedgwicki, ■ is 

 distinguished from other species of the same genus by the nutri- 

 tive blastodermic vessicle on the head of its embryos, and by 

 the different stages of the embryo found in the same female. 



