STANDARD - Survey Instrument Antique Center!

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case the real aperture of the telescope is found by moving the pointer over the objectglass
until its point is just visible, and measuring from the inner edge of the brasi
cell holding the object-glass to the pointer. Twice this distance subtracted from
the distance between the two edges of the brass cell, will give the real or clear
aperture of the telescope. The clear aperture, divided by the diameter of the small
circle of light at the eye end, when the telescope is focussed on a distant object,
will give the magnifying power of the telescope. Thus the clear aperture of a telescope,
measured by means of a pair of dividers and a scale, was 1- 35, while the
diameter of the circle of light at the eye end, was, 0'- 06. In this case, the magnifying
power of the telescope was = 22.5 diameters.
-jr-
Another way to determine the magnifying power, is to measure the angular
distance between two points with a transit, and then measure the same distance
with the telescope of which the power is to be ascertained, placed so that the transit
must point into its object-glass and see the same angular distance through the
second telescope inverted. Then calling the first angle A, and the angle as seen
diminished through the introduction of the second telescope inverted a, we have
an"
the magnifying power of the second telescope = -
.
(? Thus the angle subtended
by a window sash, several hundred feet away, was measured by a transit
instrument direct, and found to be, 158' 50". When a Y level, previously focussed
on a distant object, ws set before the transit, with its object-glass towards th