The lens farthest from the eye in a compound eyepiece.
17) are the eye lens and field lens respectively of a Merz positive eye-piece.
He then altered the magnifying power by sliding the field lens of the eye-piece (which was fitted with a slipping tube for the purpose) along the eye-tube, till the images were brought into contact.
The mere size of the apparent field of view depends upon obtaining the oblique pencils of light emerging from it to cross the axis at the great possible angle, and to this end the presence of a field-lens is indispensable, which is separated from the eye-lens by a considerable interval.
4) in which the field-lens is changed into a meniscus having radii in about the ratio of +I to - 9 gives still better results, but still not quite so good as the results obtained by using the combination of two convexo-plane lenses of the focal ratio 2 to I.
The field-lens is thus in the principal focal plane of the eye-lens, if the separation be equal to -f f2).