Fig. 16.—Ramsden eye-piece.
Larger image
84.—The Field of View should be as bright as possible. To ensure this, the field of the object-glass which is taken by the eye-piece at the position of the front of the eye should not be larger than the pupil. If the whole field of light enter the eye as it should do, the brightness will then vary directly as the square of the diameter of the object-glass, and inversely as the square of the magnifying power. The directions of the rays are shown by dotted lines as aa and a′a′ for the Ramsden eye-piece in Fig. 16. This eye-piece is sometimes called an inverting eye-piece. It is not really so: the object-glass inverts its image and the eye-piece picks up the image in its inverted position. Two or three eye-pieces of this kind, of different magnifying powers, are sometimes supplied with one surveying instrument. The same form of eye-piece, being also a simple microscope, is used to read the divisions on the divided circles of theodolites, sextants, and other instruments, and for such purposes it is often desirable to ascertain its focal length.
85.—The Focal Length of the positive or Ramsden eye-piece is found by dividing the product of the focal lengths of the two lenses by their sum, diminished by the distance between them. Thus, if the focal length of each of the lenses be 1·5 inches, the distance between them 1 inch:—
1·5 × 1·5 3 - 1 = 1·125 inches.
86.—The Magnifying Power of the Telescope.—The focal length of the objective divided by that of the eye-piece gives the power of the telescope. Thus, a 14-inch telescope with the above eye-piece would have a power,
14 1·125 = 12·444, or 12½ nearly,
a very general lower power eye-piece with telescopes of this focus.