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LENSES 115
other so far as dispersion is concerned, for crown glass is more strongly convergent for blue than red rays, and flint glass is more strongly divergent for blue than red rays.
If a lens is achromatic, then the rays of light which act most strongly upon the sensitive film will come to a focus at the same point as those which have most effect upon the eye. This makes it quite easy to test whether a lens is really achromatic, for if a chequered board be placed so that its plane is at right angles to the direction of the camera, and the middle square be carefully focussed on the screen, then, when a photograph of this has been taken, if it be found that some other square has the sharpest outline, the lens cannot be said to be achromatic. This of course should be done with the lens at full aperture.
The Flare Spot. — Sometimes it may occur that a more or less pronounced central fog is always to be seen on photographs taken with a certain lens. In this case the defect is due to what is known as a flare spot in the lens, and this can very often be remedied by slightly changing the position of the stops.
In the best lenses these faults are reduced to a minimum, and, in fact, it may be taken as granted that any lens of first-class make does not possess them.
Other Faults in Lenses. — There are still one or two points of interest with respect to the glass itself to which it is worth calling attention.
All lenses used for photographic work should be carefully protected from light when not actually in use, since a prolonged exposure to light tends to give the glass a yellowish tint, and this of course must have the effect of rendering the lens much slower in its action (see also p. 67).
Very minute specks or airbells, when present in the glass, have practically no ill effect as regards the photograph produced. On the other hand, when the glass has not been