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ment of the principles upon which the Photographic art is founded。
SOLAR and Steller light contains three kinds of rays； viz：
1。 Colorific； or rays of color。
2。 Calorific； or rays of heat。
3。 Chemical rays； or those which produce chemical effects。
On the first and third the Photographic principle depends。 In explaining this principle the accompanying wood cuts； （figs。 3 and 4） will render it more intelligible。
If a pencil of the sun's rays fall upon a prism； it is bent in passing through the transparent medium； and some rays being more refracted than others； we procure an elongated image of the luminous beam； exhibiting three distinct colors； red； yellow and blue； which are to be regarded as primitivesand from their interblending； seven； as recorded by Newton； and shown in the accompanying wood cut。 These rays being absorbed； or reflected differently by various bodies； give to nature the charm of color。 Thus to the eve is given the pleasure we derive in looking upon the green fields and forests； the enumerable varieties of flowers； the glowing ruby； jasper； topaz； amethist； and emerald； the brilliant diamond； and all the rich and varied hues of nature； both animate and inanimate。 ＇hipho_3。gif＇
Now； if we allow this prismatic spectrum （b。 fig。 3。） to fall upon any surface （as at c。） prepared with a sensitive photographic compound； we shall find that the chemical effect produced bears no relation to the intensity of the light of any particular colored ray； but that； on the contrary； it is dispersed over the largest portion of the spectrum； being most energetic in the least luminous rays； and ever active over an extensive space； where no traces of light can be detected。 Fig。 4； will give the student a better idea of this principle。 It is a copy of the kind of impression which the spectrum； spoken of； would make on a piece of paper covered with a very sensitive photographic preparation。 The white space a。 corresponds with the most luminous； or yellow ray； （5； fig。 3） over limits of which all chemical change is prevented。 A similar action is also produced by the lower end of the red ray c； but in the upper portion； however we find a decided change （as at d）。 The most active chemical change； you will percieve； is produced by the rays above the yellow a； viz。 4； 3； 2 and 1 （as at b） the green （4） being the least active； and the blue （3） and violet （1） rays the most so； the action still continuing far beyond the point b which is the end of the luminous image。 ＇hipho_4。gif＇
Suppose we wish to copy by the Daguerreotype； or Calotype process； any objects highly coloredblue； red and yellow； for instance predominating the last of course reflects the most light； the blue the least； but the rays from the blue surface will make the most intense impression； whilst the red radiations are working very slowly； and the yellow remains entirely inactive。 This accounts for the difficulty experienced in copying bright green foliage； or warmly colored portraits； a large portion of the yellow and red rays entering into the composition of both and the imperfections of a Daguerreotype portrait of a person with a freckled face depends upon the same cause。
A yellow； hazy atmosphere； even when the light is very bright； will effectually prevent any good photographic result and in the height of summer； with the most sensative process； it not unfrequently happens that the most anoying failures arise from this agency of a yellow medium。 A building painted of a yellow color； which may reflect the sun's rays directly into the operator's room will have the same effect。 Daguerreotypists； being ignorant of these facts； are very apt to charge their want of success to the plates； or chemicals； or any thing but the real cause； and it would be well to bear these facts constantly in mind and as far as possible avoid them。 This； may be accomplished； in a measure； by a choice of location or by having the glass of your windows tinged with blue； or a screen of thin blue paper may be interposed between the light and sitter。 In selecting subjects； all striking contrasts in color should be avoided； and sitters for portraits should be cautioned not to wear anything that may produce the effect spoken of dark dresses always being the best。
The action of light both combines and decomposes bodies。 For instance； chlorine and hydrogen will remain in a glass vessel without alteration if kept in the dark； but if exposed to the rays of the sun； they immediately enter into combination； and produce hydrochloric acid。 On the other hand； if colorless nitric acid be exposed to the sun； it becomes yellow； then changes to red； and oxygen is liberated by the partial decomposition effected by the solar rays。
Of the organic substances none are more readily acted upon by light than the various combinations of silver。
Of these some are more； and others less sensitive。 If Chloride of silver； which is a white precipitate formed by adding chloride of sodium （common salt） to a solution of nitrate of silver； be exposed to diffused light； it speedily assumes a violet tint； and ultimately becomes nearly black。 With iodide of silver； bromide of silver； ammonio…nitrate of silver； and other salts of this metal； the result will be much the same。
Some bodies； which under the influence of light； undergo chemical changes； have the power of restoring themselves to their original condition in the dark。 This is more remarkably displayed in the iodide of platinum； which readily recieves a photogenic image by darkening over the exposed surfaces； but speedily loses it by bleaching in the dark。 The ioduret of Daguerre's plate； and some other iodides； exhibit the same peculiarityThis leads us to the striking fact； that bodies which have undergone a change of estate under the influence of day…light have some latent power by which they can renovate themselves。 Possibly the hours of night are as necessary to inanimate nature as they are to the animate。 During the day； an excitement which we do not heed； unless in a state of disease； is maintained by the influence of light and the hours of repose； during which the equilibrium is restored； are absolutely necessary to the continuance of health。
Instead of a few chemical compounds of gold and silver； which at first were alone supposed to be photographic； we are now aware that copper； platinum； lead； nikel； and indeed； probably all the elements； are equally liably to change under the sun's influence。 This fact may be of benefit to engravers； for if steel can be made to take photographic impressions； the more laborious process of etching may be dispensed with。 In fact； in the latter part of this work； a process is described for etching and taking printed impressions from Daguerreotype plates。 As yet this process has produced no decided beneficial results but future experiments may accomplish some practical discovery of intrinsic value to the art of engraving。
A very simple experiment will prove how essential light is to the coloring of the various species comprising the vegetable and animal kingdoms。 If we transplant any shrub from the light of day into a dark cellar； we will soon see it lose its bright green color； and be