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88 THE PHOTOGRAPHIC NEWS. [Feb. 21,1862. Feb, terious nature of the gas, and also of its action on metals and the different chemicals, especially silver salts, met with in the photographic laboratory. In fact, so injurious is the pre sence of this gas to the ordinary photographic processes, that the photographer who has not another room in which to perform such operations as these, except the one in which he prepares his sensitive plates and paper, should on no account attempt to prepare sulphuretted hydrogen, but either pur chase' its aqueous solution ready made (and even this will require great care in using), or dispense with it altogether. Hydrosulphuric acid is a colourless gas, of an intensely disagreeable odour of rotten eggs, very poisonous in its undiluted state, and injurious even when dilute, producing fainting and asphyxia, and acting like a narcotic poison. It burns, like hydrogen, with a blue flame, forming water and sulphurous acid, with deposition of sulphur. It is soluble in water when passed through it, and as this aqueous solution is the one most frequently of use to the photogra phic chemist, we have given above the method of forming it. The bottle should from time to time be removed from the evolution tube, and having inserted the stopper, be well shaken; if, upon removing the stopper, the contents seem to suck inwards, it is a sign that the water is not yet satu rated, and it must be submitted to the stream of gas once more, the generation being kept up in the large bottle by adding dilute sulphuric acid from time to time; if however the stopper, upon being removed, appears to be blown out, it is a sign that the water is saturated with gas, when the bottle must be carefully stoppered and placed aside for use. It is advisable, if the stopper be well ground in, to allow the bottle to stand upside down in a corner, its weight resting on the stopper; this prevents the possibility of any air being absorbed by the aqueous solution. In this case the stopper, before being put in, should be well lubricated by being warmed and then rubbed over with solid paraffin. The amount of sulphuretted hydrogen gas which water will absorb is from two and a half to three times its volume. The solution forms a colourless liquid, having the odour of the gas, which, when exposed to the air, or heated, is en tirely evolved. Upon standing, the solution absorbs oxygen from the air, which unites with the hydrogen of the sulphu retted hydrogen, forming water, with precipitation of sulphur, which gives the liquid a milky appearance :— HS+O=HO+S. The chemical reactions of hydrosulphuric acid, and the uses to which it may be applied in the photographic labo ratory, must be deferred to our next article. PROFESSOR TYNDALL’S LECTURES ON LIGHT. LECTUEE iv.—Jan. 2, 1862. With regard to the duration of the impression upon the retina, Professor Tyndall commenced by illustrating a very simple and beautiful form of experiment. The illustration was effected by means of the electric light, an image of the effect produced being projected on the screen; but the same thing, the lecturer explained, could be tried by each of his audience in the following manner:—Take a knitting needle, stick a little silvered bead on the top of it by means of marine glue or sealing wax, and having fastened the other end firm in a vice, strike it so as to cause it to vibrate. When the sunlight, or even the light of a lamp or candle is allowed to fall upon the bead upon striking it, the needle vibrates, and the bead goes on describing the most beautiful figures. Generally there is formed a figure of 8, which denotes the vibrations of which the rod is capable; at other times, the spot of light forms the most beautiful circles and ovals, with crimped or serrated edges, and every moment changing into some new form. Several popular toys depend upon this principle of the persistence of vision, amongst others being the chromatrope. The subject of irradiation was then mentioned. This effect is produced when the eye looks at a luminous object; between these two. carried still further by taking the coloured rays forming the RI mi itse white shouli would The variot to wli mains has b dered is usi its or exhib placir the e. means The I exhib it was specti each specti The 1 the s: perat Th meta and This plain can vapoi trie 1 elect) proje and sodit insts the, T brie: to b itsel surr rays or j the spei line pro oft extremity of the spectrum. Instantly the red flowers glowed with increased brilliancy, whilst the green leaves were per fectly black. Upon passing them along, as the bunch approached the green rays, the flowers grew black, and the leaves shone out with their natural colour, and when they left the green, and arrived at the blue rays, the whole bunch appeared jet black. The explanation of this is, that the colouring matter of the red leaves has the power of com pletely quenching, absorbing, drinking in, and destroying the blue, green, and yellow, whilst the only light which it ployed instead of the glass one to disperse the light, when the colours were seen to be pulled more asunder, and a more < richly coloured spectrum was produced. Another bisulphide I of carbon prism was then interposed in the course of the rays I after they had undergone dispersion by the first prism, when the spectrum was greatly increased in length, stretching entirely across the screen. This was employed to explain the production of colours in nature. A bunch of red arti ficial flowers with green leaves, was introduced into the red spectrum, and actually building them up again into the ) form of the very coal points from which they originally issued. Newton's experiment of the blending together of the seven primary colours into white light was then per formed by projecting the image of a glass circle, painted with transparent colours, upon the screen. It was caused to rotate rapidly, when the different colours vanished, forming white light. The spectrum which had hitherto been employed to illus trate these facts was from a glass prism; and attention was drawn to the width which the colours were drawn apart when this material was used. Some substances, it was stated, had the power of drawing the colours more widely apart than others; glass, for example, does this more effectually than water, and bisulphide of carbon more effectually than glass. This drawing asunder of the colours by a prism was called disper sion : thus the greater the distance between the red and violet ends of the spectrum, the greater is the dispersion. A hollow glass prism filled with bisulphide of carbon was then em is capa afterwa Ander plexioi spectrt with tl one sic becam Upon radian j until, splenc I entire The spectrum of the electric light was then formed, and it was shown that by placing a lens in front of the prism, the colours could be so blended together again as to reproduce white light. The illustration was it appears larger than it really is, and the more intense the light, the larger does the object appear. Thus the full moon appears larger when looked at with the naked eye than when looked at through a dark glass. The bright new moon appears also to belong to a larger sphere than the dusky globe which it partially encircles. The lecturer illus trated this by exhibiting two rings of exactly the same size; one being black on a white ground, and the other white on on a black ground. Upon illuminating this with the elec tric light, the white ring appeared considerably the larger of the two, and even in the ordinary light of the room there appeared a slight difference in size. Another illustration of the same subject was afforded by a fine platinum wire ignited white hot by the battery; it appeared considerably thicker in that state than when it was cold, or when it was looked at through a coloured glass. The special subject of the day’s lecture was then com menced. A slice of white light was first of all projected on to the screen, and then a little glass prism was interposed in its path. The beam was bent very much on one side, and the white light was reduced to its coloured components. This is the grand discovery of the great Newton. The white light of the sun was then explained to be made up of an infinite number of rays of different refrangibilities. Each particular refrangibility corresponds to a particular colour ; hence the number of colours involved in solar light is in finite ; but for convenience sake, we divide these colours into seven, which are called primary colours. These are Red, Orange, Yellow, Green, Blue, Indigo, and Violet. Of these colours the red is the least refrangible, and the violet the most refrangible; the other colours being intermediate
