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Now, as has been said before, the dissimilar rays having an unequal degree of refrangibility, it will be impossible to obtain a focus by the light passing through a double-convex lens without its being fringed with color. Its effect will be readily understood by reference to the accompanying cut.
If L L be a double convex-lens, and R R R parallel rays of white light, composed of the seven colored rays,
each having a different index of refraction, they cannot be refracted to one and the same point; the red rays, being the least refrangible, will be bent to r, and the violet rays, being the most refrangible, to v: the distance v r constitutes the chromatic aberration, and the circle, of which the diameter is a l, the place or point of mean refraction, and is called the circle of least aberration. If the rays of the sun are refracted by means of a lens, and the image received on a screen placed between C and o, so as to cut the cone L a l L, a luminous circle will be formed on the paper, only surrounded by a red border, because it is produced by a section of the cone L a l L, of which the external rays L a L l, are red; if the screen be moved to the other side of o, the luminous circle will be bordered with violet, because it will be a section of the cone M a M l, of which the exterior rays are violet. To avoid the influence of spherical aberration, and to render the phenomena of coloration more evident, let an opaque disc be placed over the central portion of the lens, so as to allow the rays only to pass which are at the edge of the glass; a violet image of the sun will then be seen at v, red at r, and, finally, images of all the colors of the spectrum in the intermediate space; consequently, the general image will not only be confused, but clothed with prismatic colors."
To overcome the difficulty arising from the chromatic aberration, the optician has only to employ a combination of lenses of opposite focal length, and cut from glass possessing different refrangible powers, so that the rays of light passing through the one are strongly refracted, and in the other are bent asunder again, reproducing white light.
To the photographer one of the most important features, requiring his particular attention. is, that he be provided with a good lens. By the remarks given in the preceding pages, he will be enabled, in a measure, to judge of some of the difficulties to which he is occasionally subjected. We have in this country but two or three individuals who are giving their attention to the manufacture of lenses, and their construction is such, that they are quite free from the spherical or chromatic aberration.
To make Plates for the Daguerreotype--Determining the Time of Exposure in the Camera--Instantaneous Process for Producing Daguerreotype-- Galvanizing the Daguerreotype Plate--Silvering Solution-- Daguerreotype without Mercury--Management of Chemicals-- Hints and Cautions--Electrotyping--Crayon Daguerreotypes-- Illuminated Daguerreotypes--Natural Colors in Heliography-- Multiplying Daguerreotypes on one Plate--Deposit in Gilding-- Practical Hints on the Daguerreotype.
TO MAKE PLATES FOR THE DAGUERREOTYPE.
I do not give the method employed by our regular plate manufacturers; this is not important, as the operator could not possibly profit by it from the fact of the great expense of manufacturing. The following will be found practical:
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