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range of colors nearly corresponding with the natural hues of the prismatic spectrum。 Under favorable circumstances the mean red ray； leaves a red impression； which passes into a green over the space occupied by the yellow rays。 Above this a leaden hue is observed； and about the mean blue ray； where the action is greatest； it rapidly passes through brown into black； and through the most refrangible rays it gradually declines into a bluish brown； which tint is continued throughout the invisible rays。 At the least refrangible end of the spectrum； the very remarkable phenomenon has been observed； of the extreme red rays exerting a protecting influence； and preserving the paper from that change； which it would otherwise undergo； under the influence of the dispersed light which always surrounds the spectrum。 Not only the extreme red ray exerts this very peculiar property； but the ordinary red ray through nearly its whole length。
In photographic drawing this salt is of the utmost importance。 Mr。 Talbot's application of it will be given hereafter in another portion of this work。
IODIDE OF SILVERPerfectly pure； undergoes very little change under the influence of light or heat； but if a very slight excess of the nitrate of silver be added it becomes infinitely more senitive than the chloride
The spectrum impressed upon paper prepared with a weak solution of the hydriodate of potash presents some very remarkable peculiarities。 The maximum of intensity is found at the edge of the most refrangible violet rays； or a little beyond it； varying slightly according to the kind of paper used； and the quantity of free nitrate of silver present。 The action commences at a point nearly coincident with the mean red of the luminous spectrum； where it gives a dull ash or lead color； while the most refrangible rays impress a ruddy snuff…brown； the change of tint coming on rather suddenly about the end of the blue or beginning of the violet rays of the luminous spectrum。 Beyond the extreme violet rays； the action rapidly diminishes； but the darkening produced by these invisible rays； extends a very small space beyond the point at which they cease to act on the chloride of silver。
In its photographic application； it is； alone； of very little use； but in combination with other reagents it becomes exquisitely sensitive。 With gallic acid and the ferrocyanate of potash it forms two of the most sensitive photographic solutions with which we are acquainted。 These are used in the calotype process。
IODURET OF SILVER。If upon a plate of polished silver we place a small piece of iodine； and apply the heat of a lamp beneath the plate for a moment； a system of rings is speedily formed。 The first ring； which spreading constantly forms the exterior of the circle； is of a bright yellow color； within this； there arises； sucessively； rings of green； red and blue colors； and then again a fine yellow circle； centred by a greyish spot on the place occupied by the iodine。 On exposing these to the light； the outer yellow circle almost instantly changes color； the others slowly； in the order of their position； the interior yellow circle resisting for a long time the solar influence。 These rings must be regarded as films of the ioduret of silver； varying； not only in thickness； but in the more or less perfect states of combination in which the iodine and metal are。 The exterior circle is an ioduret in a very loose state of chemical agregation； the attractive forces increase as we proceed towards the centre； where a well formed ioduret； or probably a true iodide of silver； is formed； which is acted upon by sunlight with difficulty。 The exterior and most sensitive film constitutes the surface of Daguerreotype plates。 The changes which these colored rings undergo are remarkable； by a few minutes exposure to sunlight； an inversion of nearly all the colors takes place； the two first rings becoming a deep olive green； and a deep blue inclining to black。
The nature of the change which the ioduret of silver undergoes on Daguerreotype plates； through the action of light； Mr。 Hunt considers to be a decided case of decomposition； and cites several circumstances in proof of his position。 These with other facts given by Mr。 Hunt in his great work on the Photographic art； but to volumnious to include in a volume of the size to which I am obliged to cofine myself； should be thoroughly studied by all Daguerreotypists。
PRISMATIC ANALYSIS。The most refrangible portion of the spectrum； （on a Daguerreotype plate） appears； after the plate has been exposed to the vapor of mercury； to have impressed its colors； the light and delicate film of mercury； which covers that portion； assuming a fine blue tint about the central parts； which are gradually shaded off into a pale grey； and this is again surrounded by a very delicate rose hue； which is lost in a band of pure white。 Beyond this a protecting influence is powerfully exerted； and notwithstanding the action of the dispersed light； which is very evident over the plate； a line is left； perfectly free from mercurial vapor； and which； consequently； when viewed by a side light； appears quite dark。 The green rays are represented by a line of a corresponding tint； considerably less in size than the luminous green rays。 The yellow rays appear to be without action； or to act negatively； the space upon which they fall being protected from the mercurial vapor； and it consequently is seen as a dark band。 A white line of vapor marks the place of the orange rays。 The red rays effect the sensitive surface in a peculiar manner； and we have the mercurial vapor； assuming a molecular arrangement which gives to it a fine rose hue； this tint is surrounded by a line of white vapor； shaded at the lowest extremity with a very soft green。 Over the space occupied by the extreme red rays； a protecting influence is again exerted； the space is retained free from mercurial vapor and the band is found to surround the whole of the least refrangible rays； and to unite itself with the band which surrounds the rays of greatest refrangibility。 This band is not equally well defined throughout its whole extent。 It is most evident from the extreme red to the green； it fades in passing through the blue； and increases again； as it leaves the indigo； until beyond the invisible chemical rays it is nearly as strong as it is at the calorific end of the spectrum。
Images on Daguerreotype plates which have been completely obliterated by rubbing may be restored； by placing it in a tolerably strong solution of iodine in water。
BROMIDE OF SILVER。This salt； like the iodide； does not appear to be readily changed by the action of light； but when combined with the nitrate of silver it forms a very sensitive photographic preparation。
Paper prepared with this salt； blackens over its whole extent with nearly equal intensity； when submitted to the prismatic spectrum。 The most characteristic peculiarity of the spectrum is its extravagant length。 Instead of terminating at the mean yellow ray； the darkened portion extends down to the very extremity of the visible red rays。 In tint it is pretty uniformly of a grey…black over its whole extent； except that a slight fringe of redness is