The Theory of Modern Optical Instruments; A Reference Book for Physicists, Manufacturers of Optical Instruments, and for Officers in the Army and Navy

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1921 Excerpt: ... Where Hf) is the natural illumination (i.e., with the unaided eye), p is the radius of the exit pupil of the instrument, and pa is the radius of the entrance pupil of the eye. The illumination H thus increases as p increases (assuming that p and H0 are constant) until p becomes equal to pa. Neglecting losses of light by reflection and absorption, the illumination through the telescope is then the same as with the unaided eye. In this case the pupil of the eye is completely filled with rays, and further increase of p serves no purpose. In darkness the pupil dilates, and thus pa is a variable quantity. In ordinary daylight pa has a value of 1-5 mm.; consequently all telescopes with exit pupil of diameter 3 mm. or over, have normal illumination. The pupil of the eye in twilight has a diameter of 7 mm. and therefore only those telescopes whose exit pupils are 7 mm. in diameter possess normal brightness when used during twilight. Modern prismatic telescopes, especially those required to be easily carried by hand, are constructed so that the exit pupil is.3-5 mm. in diameter, whilst Galilean telescopes (opera glasses) usually have an exit pupil of 7 mm. and over. From this point of view, the latter are termed "night glasses." Ex. 100. At night the radius pa of the pupil of the eye is 4 mm. A prismatic telescope which magnifies five times and in which p = 1-5 mm. is compared with a Galilean telescope in which p = 5 mm. and magnification of x 5. What is the ratio of their illuminations and what are the sizes of their objectives? 29129 L 2 The effective exit pupil of the Galilean telescope has a radius of 4 mm. From equation (11) the illumination in the two telescopes under the existing circumstances are proportional to the squares of the radii of ...