the history and practice of the art of photography-第22章
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Most flowers give out their coloring matter to alcohol or water。 Some； however； refuse to do so； and require the addition of alkalies； others of acid； &c。 Alcohol has； however； been found to enfeeble； and in many cases to discharge altogether these colors； but they are； in most cases； restored upon drying； when spread over paper。 Papers tinged with vetegable colors must always be kept in the dark； and perfectly dry。
The color of a flower is by no means always； or usually； that which its expressed juice imparts to white paper。 Sir John Herschel attributes these changes to the escape of carbonic acid in some cases； to a chemical alteration； depending upon the absorption of oxygen； in others； and again in others； especially where the expressed juice coagulates on standing； to a loss of vitality； or disorganization of the molecules。 To secure an eveness of tint on paper； the following manipulation is recommended：The paper should be moistened on the back by sponging and blotting off。 It should then be pinned on a board； the moist side downwards； so that two of its edges （suppose the right…hand and lower ones） shall project a little beyond those of the board。 The board then being inclined twenty or thirty degrees to the horizon； the alcoholic tincture （mixed with a very little water； if the petals themselves be not very juicy） is to be applied with a brush in strokes from left to right； taking care not to go over the edges which rest on the board； but to pass clearly over those that project； and observing also to carry the tint from below upwards by quick sweeping strokes； leaving no dry spaces between them； but keeping up a continuity of wet spaces。 When all is wet； cross them by another set of strokes from above downwards； so managing the brush as to leave no floating liquid on the paper。 It must then be dried as quickly as possible over a stove； or in a warm current of air； avoiding； however； such heat as may injure the tint。
In addition to the flowers already mentioned in my third chapter； the following are among those experimented upon and found to give tolerable good photographic sensitives。 I can only enumerate them； referring the student； for any further information he may desire on the subject； to Mr。 Hunt's work； although what I have said above is sufficient for all practical purposes； and any one； with the ambition； can readily experiment upon them； without further research； on any other flower he may choose。
Viola Odorataor sweet sented violet； yields to alcohol a rich blue color； which it imparts in high perfection to paper
Senecio Splendensor double purple groundsel； yields a beautiful color to paper。
The leaves of the laurel； common cabbage； and the grasses； are found sufficiently senstive。
Common Merrigold yields an invaluable faecula； which appears identical with that produced by the Wall…flower； and Cochorus japonica mentioned before； and is very sensitive； but photographs procured upon it cannot be preserved； the color is so fugitive。
From an examination of the researches of Sir John Herschel on the coloring matter of plants； it will be seen that the action of the sun's rays is to destroy the color； effecting a sort of chromatic analysis； in which two distinct elements of color are separated； by destroying the one and leaving the other outstanding。 The action is confined within the visible spectrum； and thus a broad distinction is exhibited between the action of the sun's rays on vegetable juices and on argentine compounds； the latter being most sensibly affected by the invisible rays beyond the violet。
It may also be observed； that the rays effective in destroying a given tint； are in a great many cases； those whose union produces a color complementary to the tint destroyed； or； at least； one belonging to that class of colors to which such complementary tint may be preferred。 For instance； yellows tending towards orange are destroyed with more energy by the blue rays； blues by the red； orange and yellow rays； purples and pinks by yellow and green rays。
V。 AMPHITYPE。
This process is a discovery of Sir John Herschel and receives its name from the fact that both negative and positive photographs can be produced by one process。 The positive pictures obtained by it have a perfect resemblance to impressions of engravings with common printer's ink。 The process； although not yet fully carried out； promises to be of vast utility。
Paper proper for producing an amphitype picture may be prepared either with the ferro…tartrate or the ferro…citrate of the protoxide； or the peroxide of mercury； or of the protoxide of lead； by using creams of these salts； or by successive applications of the nitrates of the respective oxides； singly or in mixture； to the paper； alternating with solutions of the ammonia…tartrate or the ammonia…citrate of iron； the latter solution being last applied； and in more or less excess。 I purposely avoid stating proportions； as I have not yet been able to fix upon any which certainly succeed。 Paper so prepared and dried takes a negative picture； in a time varying from half an hour to five or six hours； according to the intensity of the light； and the impression produced varies in apparent force from a faint and hardly perceptible picture to one of the highest conceivable fulness and richness both of tint and detail； the color being in this case a superb velvety brown。 This extreme richness of effect is not produced unless lead be present； either in the ingredients used； or in the paper itself。 It is not； as I originally supposed； due to the presence of free tartaric acid。 The pictures in this state are not permanent。 They fade in the dark； though with very different degrees of rapidity； some （especially if free tartaric or citric acid be present） in a few days； while others remain for weeks unimpaired； and require whole years for their total obliteration。 But though entirely faded out in appearance； the picture is only rendered dormant； and may be restored； changing its character from negative to positive； and its colors from brown to black； （in the shadows）； by the following process：A bath being prepared by pouring a small quantity of solution of pernitrate of mercury into a large quantity of water； and letting the subnitrated precipitates subside； the picture may be immersed in it； （carefully and repeatedly clearing off all air bubbles；） and allowed to remain till the picture （if any where visible；） is entirely destroyed； or if faded； till it is judged sufficient from previous experience； a term which is often marked by the appearance of a feeble positive picture； of a bright yellow hue； on the pale yellow ground of the paper。 A long time （several weeks） is often required for this； but heat accelerates the action； and it is often completed in a few hours。 In this state the picture is to be very thoroughly rinsed and soaked in pure warm water； and then dried。 It is then to be well ironed with a smooth iron； heated so as barely not to injure the paper； placing it； for greater security against scorching； between clean smooth paper。 If then the process have been successful； a perfectly black positive picture is at once developed。 At first it most commonly happens that the whole picture is sooty or dingy to such a degree that it is condemned as spoiled； but on keeping it between the leaves of a book； especially in a moist atmosphere； by extremely slow degrees this dinginess disappears； and the picture disengages itself with continually increasing sharpness and clearness； and acquires the exact effect of a copper…plate engraving on a paper more or less tinted with a pale yellow。
I ought to observe； that the best and most uniform specimens which I have procured have been on paper previously washed with certain preparations of uric acid； which is a very remarkable and powerful photographic element。 The intensity of the original negative picture is no criterion of what may be expected in the positive。 It is from the production by one and the same action of light； of either a positive or negative picture according to the subsequent manipulations； that I have designated the process； thus generally sketched out； by the term Amphitype；a name suggested by Mr。 Talbot； to whom
