The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Chromatic Aberration
where the different coloured rays cross the axis, though for the sake of clearness these are not shown. Now the brightest part of the spectrum to the human eye is in the yellow, whilst the rays which act most on the plate are in the blue and violet, so that if we obtain an image by the yellow rays, the plate would be affected by the blue and violet, and we should get no sharp picture, because at Y the blue rays would be a circle instead of a point. This is termed non-coincidence of the visual and chemical foci, or the lenses are said to be uncorrected or non-achromatic. Now it is a fact that crown and flint glass have different dispersive powers, that is, they produce spectra which differ in theposition of the Fraunhofer lines. With the crown glass the distances between A b and b c, in the red and orange, is greater than with the flint, whereas with the flint the distances
Fig. 23.
between f g and G H are greater than with the crown. Therefore we may assert that, with the crown glass, the red end or [less refrangible rays are extended at the expense of the violet or more refrangible rays, whilst with the flint glass the opposite holds good. We have seen that we have a lens which is practically a collection of prisms; we have also seen that the visual and chemical rays do not have the same focus, and it is essential they should possess this in common. Light is always refracted towards the base of a prism, and we have just pointed out that flint and crown glass act differently on the spectrum; the one extends the blue, the other the red end more, so that it is not difficult to see from fig. 23, that what we want to do is to make /fall upon /'. This represents, exaggerated of course, the action of a convex crown-glass lens. Now, suppose we examine the action of a concave flint-glass lens. In this case, fig. 24, it is seen that the blue rays are bent out more than
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