A1.1.4 The angle subtended by a line of length d, in a plane
perpendicular to the line of sight, and separated from the eye of
the observer by a distanceD, is given in degrees by 57.3d/D.
The angle subtended by the image of this line, seen by viewing
it through an eyepiece of magnification M, is given in degrees
by 57.3Md/D i, whereDi is the distance between the eye of the
observer and the plane of the image.