10 REPORT — 1856. 



bent in contrary directions ; but by inclining gradually the optical axes, we can 

 converge them until the two images coalesce, and we see only one window; as soon 

 as they coincide the lateral curvature of the vertical lines ceases, and they are bent 

 projectively from back to front : we have then the illusion of a window concave 

 towards the room, such as it would appear reflected by a concave mirror. 



On some Dichromatic Phcenomena among Solutions, and the means of representing 

 them. By J. H. Gladstone, Ph.D., F.R.S. 

 This paper was an extension of Sir John Herschel's observations on dichromatism, 

 that property whereby certain bodies appear of a different colour according to the 

 quantity seen through. It depends generally on the less rapid absorption of the red 

 ray as it penetrates a substance. A dichromatic solution was examined by placing 

 it in a wedge-shaped glass- trough, held in such a position that a slit in a window- 

 shutter was seen traversing the varying thicknesses of the liquid. The diversely 

 coloured line of light thus produced was analysed by a prism ; and the resulting 

 spectrum was represented in a diagram by means of coloured chalks on black paper, 

 the true position of the apparent colours being determined by the fixed lines of the 

 spectrum. In this way the citrate and comenamate of iron, sulphate of indigo, 

 litmus in various conditions, cochineal, and chromium, and cobalt salts were examined 

 and represented. Among the more notable results were the following : — A base, 

 such as chromic oxide, produces very nearly the same spectral image with whatever 

 acid it may be combined, although the salts may appear very different in colour to 

 the unaided eye. Citrate of iron appears green, brown, or red, according to the 

 quantity seen through. It transmits the red ray most easily, then the orange, then 

 the green, while it cuts off entirely the more refrangible half of the spectrum. 

 Neutral litmus appears blue or red, according to the strength or depth of the solu- 

 tion. Alkalies cause a great development of the blue ray ; acids cause a like increase 

 of the orange, while the minimum of luminosity is altered to a position much nearer 

 the blue. Boracic acid causes a development of the violet. Alkaline litmus was 

 exhibited so strong that it appeared red, and sUghtly acid litmus so dilute that it 

 looked bluish purple ; indeed, on account of the easy transmissibility of the orange 

 ray through an acid solution, the apparent paradox was maintained that a large 

 amount of alkaline litmus is of a purer red than acid litmus itself. Another kind 

 of dichromatism was examined, dependent not on the actual quantity of coloured 

 material, but on the relative proportion of the solvent, and diagrams of the changing 

 appearances of sulphocyanide of iron, of chloride of copper, and of chloride of cobalt 

 were exhibited. 



On the Stratified Appearance of the Electrical Discharge. 

 By W. R. Grove, M.A., F.R.S. 

 Mr. Grove communicated some additional facts connected with a phaenomenon 

 first observed and published by him in the 'Philosophical Transactions' for 1852, 

 viz. the striated or stratified appearance in the electric discharge in rarefied gases 

 and vapours, particularly that of phosphorus. M. Ruhmkorff, M. Quet, and 

 Dr. Robinson had, subsequently to Mr. Grove, experimented on the subject. No sa- 

 tisfactory rationale of it has hitherto been given. Mr. Grove has, however, observed 

 that the mode of breaking contact has a marked influence on the phaenomenon, which 

 would lead to the belief that it is due to the intermittent character of the discharges. 

 If,for instance, the arm of thecontact-breaker be made to rest on a slight spring placed 

 underneath it, the bands become narrower. If a single breach of contact be effected, 

 most observers have remarked that the effect is still perceptible ; but it is very diffi- 

 cult to effect a single breach of contact. The fusion of the metals at the point of 

 contact, with the vibration accompanying the movement, occasions a double or 

 triple disruption. The best mode is to place two stout copper wires across each 

 other, and with a firm hand draw one over the other, until the end of the former 

 parts company with the latter ; when this is well done the striae are, in the majority 

 of cases, not observed. Of all the substances which had been tried, the vapour of 

 phosphoiTis succeeds best, and with this is seen a remarkable effect on the powder 

 or smoke of allotropic phosphorus (which is always formed when the striae are 



