The Dictionary Of Photography
A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.
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Astigmatism
not by solution, but by a breaking up of the exposed bitumen into small scales or grains. (See The Amateur Photographer, October 28th, 1898, p. 854.)
Astigmatism
 a dot or point).A defect in lenses from which vertical and horizontal lines near the margin of the field cannot be both accurately focussed at |
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the same time. It is particularly noticeable*with lenses of large aperture, and is cured to some extent by the use of small diaphragms or by special selection of the glass and calculation of the curves. This should not be confounded with curvature of the field, as in this case the alteration of the focus will improve the marginal definition, though destroying the focus in the centre. To explain this we must have recourse to fig. 7. If we assume / o m n to be a convex lens, and a b its axis, and
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