The first principles of photography : an elementary treatise on the scientific principles upon which practical photography depends (1892)
Leaper, Clement J., author · London : Iliffe & Son, 3 St. Bride street, E.C. · leaf 101
le at which this occurs is evidently, under the conditions under which the experiment was made, half the solid angle included by the field of visibility. Measuring the semi-angle, I find it to be about 50°, the angle itself being, therefore, 100°. Now, inserting this small stop in front of the… … Fig. 37. — Influence of Diameter of Stop upcn Field of Visibility of a Lens. Calling N the nodal point of emission (fig. 37) of the lens, then all rays A B C JD pass through it. Now if the lens is used with- out any stop evidently the angle A N B is greater than C N D, that
