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.
Valz pointed out that the other optical conditions could be equally satisfied if the divided lens were made concave instead of convex, with the advantage of giving a larger field of view (Monthly Notices, vol.
Then if the prism P4 is cemented to P3, a sharp image of such lines of the solar spectrograph as are visible in the field of view will be seen in the eyepiece.
Instruments have been invented by Alvan Clark and Sir Howard Grubb for measuring with the spider-line micrometer angles which are larger than the field of view of the eyepiece.
While the limiting of the pencil is almost always effected by the objective, the limiting of the field of view is effected by the eyepiece, and indeed it is carried out by a real diaphragm DD arranged in the plane of the real image O'O 1 (fig.
When the star enters the field of view its image is approximately bisected by the spider web of the micrometer n, the exact bisection being completed in the immediate neighbourhood of the meridian.
The placing of the analyser near the objective has the advantage that the field of view is not restricted, as is the case if the analyser .is used above the eyepiece.
Thus, any fixed telescope directed towards the mirror of a properly adjusted coelostat in motion will show all the stars in the field of view at rest; or, by rotating the polar axis independently of the clockwork, the observer can pass in review all the stars visible above the horizon whose declinations come within the limits of his original field of view.
The following may be regarded as typical: - (t) Largest aperture; necessary corrections are - for the axis point, and sine condition; errors of the field of view are almost disregarded; example - highpower microscope objectives.
Directing the finder to the comet, he has at once in the field of view all available comparison stars.