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ch； 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 perceptible at the least refracted end。 Beyond the red ray； an extended space is protected from the agency of the dispersed light； and its whiteness maintained； thus confirming the evidence of some chemical power in action； over a space beyond the luminous spectrum； which corresponds with the rays of the least refrangibility。
This salt is extensively used in photographic drawing。
PREPARATIONS OF GOLD。Chloride of Gold； freed from an excess of acid is slowly changed under the action of light； a regularly increasing darkness taking place until it becomes purple； the first action of the light being to whiten the paper； which； if removed from the light at this stage； will gradually darken and eventually develope the picture。 This process may be quickened by placing the paper in cold water。
Chloride of gold with nitrate of silver gives a precipitate of a yellow brown color。 Paper impregnated with the acetate of lead； when washed with perfectly neutral chloride of gold； acquires a brownish…yellow hue。 The first impression of light seems rather to whiten than darken the paper； by discharging the original color； and substituting for it a pale greyish tint； which by slow degrees increases to a dark slate color； but if arrested； while yet； not more than a moderate ash grey； and held in a current of steam； the color of the parts acted upon by light and of that onlydarkens immediately to a deep purple。
Here I must leave the subject of the action of light upon metalic compounds referring to Mr。 Hunts work for any further information the student may desire on the other metalsas I find myself going beyond my limits。 I cannot； however； entirely dismiss the subject without giving a few examples of the action of light on the juices of plants； some of which produce very good photographic effect。
CORCHORUS JAPONICAThe juice of the flowers of this plant impart a fine yellow color to paper； and； so far as ascertained； is the most sensitive of any vegetable preparation； but owing to its continuing to change color even in the dark； photographic images taken on paper prepared with it soon fade out。
WALL FLOWER。This flower yields a juice； when expressed with alcohol； from which subsides； on standing； a bright yellow finely divided faecula； leaving a greenish…yellow transparent liquid； only slightly colored supernatant。 The faecula spreads well on paper； and is very sensitive to light； but appears at the same time to undergo a sort of chromatic analysis； and to comport itself as if composed of two very distinct coloring principles； very differently affected。 The one on which the intensity and sub…orange tint of the color depends； is speedily destroyed； but the paper is not thereby fully whitened。 A paler yellow remains as a residual tint； and this on continued exposure to the light； slowly darkens to brown。 Exposed to the spectrum； the paper is first reduced nearly to whiteness in the region of the blue and violet rays。 More slowly； an insulated solar image is whitened in the less refrangible portion of the red。 Continue the exposure； and a brown impression begins to be percieved in the midst of the white streak； which darkens slowly over the region between the lower blue and extreme violet rays。
THE RED POPPY yields a very beautiful red color； which is entirely destroyed by light。 When perfectly dried on paper the color becomes blue。 This blue color is speedily discharged by exposure to the sun's rays； and papers prepared with it afford very interesting photographs。 Future experiments will undoubtedly more fully develope the photogenic properties of flowers； and practically apply them。
Certain precautions are necessary in extracting the coloring matter of flowers。 The petals of fresh flowers； carefully selected； are crushed to a pulp in a mortar； either alone or with the addition of a litte alcohol； and the juice expressed by squeezing the pulp in a clean linen or cotton cloth。 It is then to be spread upon paper with a flat brush； and dried in the air。 If alcohol be not added； it must be applied immediately； as the air changes or destroys the color instantly。
Most flowers give out their coloring matter to alcohol or water but the former is found to weaken； and in some cases to discharge altogether these colors； but they are in most cases restored in drying。 Paper tinged with vegetable colors must be kept perfectly dry and in darkness。
To secure an eveness of tint on paper it should be first moistened on the back by sponging； and blotting off with bibulous paper。 It should then be pinned on a board； the moist side downwards； so that two of its edges the right and lower onesproject a little over those of the board。 Incline the board twenty or thirty degrees to the horizon； and apply the tincture with a brush in strokes from right to left； taking care not to go over the edges which rests on the board； but to pass clearly over those that project； and also observing to carry the tint from below upwards by quick sweeping strokes； leaving no dry spaces between them。 Cross these with other strokes from above downwards； leaving no floating liquid on the paper。 Dry as quickly as possible； avoiding； however； such heat as may injure the tint
CHAP。 IV。
A FEW HINTS AND SUGGESTIONS TO DAGUERREOTYPISTS。
There are very few who may not be capable of practising the Photographic art； either on paper； or metalic platesbut； like all other professions； some are more clever in its various processes than others。
Impatience is a great drawback to perfect success； and combined with laziness is a decided enemy。 Besides this； no one can excel in Photography who does not possess a natural taste for the fine arts； who is not quick in discerning grace and beautyis regardless of the principles of perspective； foreshorting and other rules of drawing； and who sets about it merely for the sake of gainwithout the least ambition to rise to the first rank； both in its practice and theory。 There is no profession or trade in which a slovenly manner will not show itself； and none where its effects will be more apparent than this。
In order to be great in any pursuit； we must be ourselves； and keep all things； in order。 In your show and reception rooms； let neatness prevail； have your specimens so placed leaning slightly forwardas to obtain the strongest light upon them； and at the same time prevent that glassiness of apearance which detracts so materially from the effect they are intended to produce。 If possible； let the light be of a north…western aspect； mellowed by curtains of a semitransparent hue。 Your show…cases； at the door； should be kept well cleaned。 I have often been disgusted while attempting to examine portraits in the cases of our artists； at the greasy coating and marks of dirty fingers upon the glass and frame enclosing them。 Believe it； many a good customer is lost for no other reason。
In your operating room； dust should be carefully excluded。 It should be furnished with nothing apt to collect and retain dust； a carpet is therefore not only a useless article； but very improper。 A bare floor is to be prefered； but if you