The Elementary Theory of the Symmetrical Optical Instrument (Paperback)

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. 1908 Excerpt: ...corrected for aberration, and there must be some more elementary reason why they were constructed of two lenses when a single lens might have sufficed. For a telescope in ideal adjustment there is, of course, no equivalent thin lens. For the power is zero, and the unit planes are at infinity. To give such relative translation to the object and image systems as shall bring these unit planes into coincidence, and then to use a lens of zero power, is a proceeding which can hardly be regarded as practicable. But the Galileo telescope, or opera-glass, as frequently used, is not a telescope in ideal adjustment, and might therefore be expected to have an equivalent thin lens capable of being used in its stead. The magnification formula 1/m = Ap, where p is the distance of the object beyond the first principal focal plane, shews that, when an instrument is used for viewing objects beyond the first principal focus, the linear magnification cannot be positive and so the image cannot be upright unless the power of the instrument is positive. So, when the opera-glass is used in this way, the power is positive and the equivalent thin lens is a diverging one. Now if we view an object AB through a diverging lens, and if A'B' be the image, and 0 the centre or unit-point of the lens, OA'A and OB'B are straight lines, and the image (if upright) is on the same side of the lens as the object, namely on the side remote from the eye; if the object is distant, the image is nearer to the eye than the object. The eye E is necessarily outside the triangle A OS, and consequently the image A'B' subtends at the eye a smaller angle than the object AB; thus there is no angular magnification, but rather angular diminution, and the lens is not serving any useful end. If only E could be on ...