Fig. 1 shows the visible spectrum as it appears to the eye; the brightest part being in the yellow space, and the light gradually shading off until it ceases to be seen. Fig. 2 represents the chemical effect produced by throwing the Spectrum upon Iodide of Silver. Observe that the darkening characteristic of chemical action is most evident in the upper spaces, where the light is feeble, and is altogether absent at the point corresponding to the bright yellow spot of the visible spectrum. The actinic and luminous spectra are therefore totally distinct from each other, and the word "Photography," which signifies the process of taking pictures by light, is in reality inaccurate.
To those who have not the opportunity of working with the Solar Spectrum, the following experiments will be useful in illustrating the Photographic value of coloured light.
Experiment I.—Take a sheet of sensitive paper prepared with Chloride of Silver, and lay upon it strips of blue, yellow, and red glass. On exposure to the sun's rays for a few minutes, the part beneath the blue glass darkens rapidly, whilst that covered by the red and yellow glass is perfectly protected. This result is the more striking from the extreme transparency of the yellow glass, giving the idea that the Chloride would certainly be blackened first at that point. On the other hand, the blue glass appears very dark, and effectually conceals the tissue of the paper from view.
Experiment II.—Select a vase of flowers of different shades of scarlet, blue, and yellow, and make a Photographic copy of them, by development, upon Iodide of Silver. The blue tints will be found to act most violently upon the sensitive compound, whilst the reds and yellows are scarcely visible; were it not that it is difficult to procure in nature pure and homogeneous tints, free from admixture with other colours, they would make no impression whatever upon the plate.
In exemplifying further the importance of distinguishing between visual and actinic rays of light, we may observe that if the two were in all respects the same. Photography must cease to exist as an Art. It would be impossible to make use of the more sensitive chemical preparations from the difficulties which would attend the previous preparation and subsequent development of the plates. These operations are now conducted in what is termed a dark room; but it is dark only in a Photographic sense, being illuminated by means of yellow light, which, whilst it enables the operator easily to watch the progress of the work, produces no injurious effect upon the sensitive surfaces. If the windows of the room were glazed with blue in place of yellow glass, then it would be strictly a "dark room," but one altogether unfitted for the purpose intended.
Another point connected with the same subject and worthy of note is—the extent to which the sensibility of the Photographic compounds is influenced by atmospheric conditions not visibly interfering with the brightness of the light. It is natural to suppose that those days on which the sun's rays are the most powerful would be the best for rapid impression, but such is not by any means the case. If the light is at all of a yellow cast, however bright it may be, its actinic power will be small.
It will also be often observed in working towards the evening, that a sudden diminution of sensibility in the plates begins to be perceptible at a time when but little difference can be detected in the brilliancy of the light; the setting sun has sunk behind a golden cloud, and all chemical action is soon at an end.
In the same manner is explained the difficulty of obtaining Photographs in the glowing light of tropical climates; the superiority Of the early months of spring over those of the midsummer; of the morning sun to that of the afternoon, etc. April and May are usually considered the best months for rapid impression in this country; but the light continues good until the end of July. In August and September a longer exposure of the plates will be required.
THE SUPERIOR SENSIBILITY OF BROMIDE OF SILVER TO COLOURED LIGHT.