The Dictionary Of Photography

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

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Chromatic Aberration
the yellow, so that, if we combine a flint concave with a crown convex, we shall get the blue rays extended more, and thus neutralise the effect of the crown to some extent. I endeavour to show this in fig. 25. The focus of the crown lens is shown at 2 for the yellow rays and at 1 for the blue ; but by adding
a concave flint, the.focus both is lengthened to Now,
there are more rays than the blue and yellow in the spectrum, and it naturally occurs to ask about the others; these are left outstanding, and at form coloured rings round the central spot or sharp focusand form what is called the secondary
Fig. 25.
spectrum. In figs. 25 and 26 we only represent two lenses, but if we use three, three rays may be combined and the lens is said to be apochromatic, and the faint outstanding rays of colour are called the residuary tertiary spectrum. The advantage of the more perfect achromatism of lenses at the present time is shown from the following statement. The ordinary gelatino-