Measurement of the Magnifying Power of a Telescope - First Method

Place the telescope at some considerable distance from a large scale, or some other well-defined object divided into a series of equal parts - the slates on a distant roof, for example. Then adjust the eye-piece so that the image seen in the telescope coincides in position with the scale itself. In doing this, remember that when the telescope is naturally focussed the image is about ten inches off; and as the eye-piece is pulled further out, the image recedes until the small image formed by the object-glass is in the principal focus of the eye-glass, when the image seen is at infinity. The required position lies between these two limits, and is attained when the image seen through the telescope with the one eye is quite distinct, while at the same time the scale, as seen directly, is distinctly seen by the other eye looking along the side of the telescope; and, moreover, the two do not appear to separate as the eyes are moved from side to side.

Then the appearance to the two eyes is as sketched in fig. 30, where the magnifying power is about 8.

The number of divisions of the scale, as seen directly, covered by one of the divisions of the image of the scale can be read off, and this gives evidently the ratio of the tangents of the two angles, pbq, PaQ, and hence the magnifying power of the telescope.

If the scale used be in the laboratory, so that its distance from the telescope can be measured, the experiment should be made at different distances. Instead of reading the number of divisions of the scale occupied by one division of the image, it is best to count those occupied by some six or eight divisions of the image and divide one number by the other.

Experiment. - Determine, at two different distances, the magnifying power of the given telescope.

Enter results thus:

Telescope No. 3.
Distance between scale and telescope: 1000 cm
Lower edge of image of division 76 is at 0 on scale.
Lower edge of image of division 69 is at 99 on scale.
Magnifying power = (99-0)/(76-69)=14.14

Distance = 500 cm.
Lower edge of image of division 72 is at 95.
Lower edge of image of division 78 is at 3.
Magnifying power = (95-3)/(78-72)=15.3