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.
This is such a practical drawback that the separation is generally 4ths or $ths of the theoretical, and then the primary image viewed by the eye piece may be rather outside the field-lens, which is a great practical advantage, especially when a reticule has to be mounted in the primary focal plane, although the edge of the field is not quite achromatic under these conditions.
It consists of a piano-convex field-lens of crown glass.
There are also other eye-pieces having the field-lens double or achromatic as well as the eye-lens.
