A cyclopaedia of physical sciences : comprising acoustics, astronomy, dynamics, electricity, heat, hydrodynamics, magnetism, philosophy of mathematics, meteorology, optics, pneumatics, statics &c. &c / by J.P. Nichol with the assistance of eminent scientific men.

  • Nichol, J. P. (John Pringle), 1804-1859.
Date:
1860
Catalogue details

Licence: Public Domain Mark

Credit: A cyclopaedia of physical sciences : comprising acoustics, astronomy, dynamics, electricity, heat, hydrodynamics, magnetism, philosophy of mathematics, meteorology, optics, pneumatics, statics &c. &c / by J.P. Nichol with the assistance of eminent scientific men. Source: Wellcome Collection.

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    erect image at are then put into a proper state for ^-isiou by the eye-glass l', that is to say, the J? i Fiff. 4. rays which emerge from each point are rendered parallel among themselves, so that they can be brought together to a corresponding point on the retina by the converging power of the lens of the eye itself. This is necessary, as will be seen by reference to Eye, where it is shown that distinct vision cannot take place except the pencils, which enter the eye, consist of rays approaching to parallelism, the power of adaptation of the eye being limited.—Such are the forms of the re- fracting telescope. By the retinements of modern workmanship, and the investigations of optical science, great improvements have been, of recent years, made on the different parts of the instru- ment, particularly with reference to the achro- matism and perfection of figure of its object-glass, as also its size, and the forms of the eye-pieces. The reader is referred to Achromatism for some information on these points. The celebrated practical optician, Frauenhofer of Munich, has erected several refractors of great size and per- fection. The Dorpat telescope is of 9 inches clear aperture in the object-glass, and 14 feet focal length. Sir James South possesses an ob- ject-glass of 11^ inches diameter; and that of Mr.Cooper of Sligo, has 12^ inches clear aperture, being the largest hitherto completed, with the exception of that recently constructed by Mr. Itoss, and erected on Wandsworth Common, near London, which has an aperture of two feet. Great practical difficulties are experienced in the con- struction of large object-glasses, not the least of Avhich is the inequality in the texture of the glass itself, no process having hitherto been discovered which can insure uniformity of refracting power over so large a mass—The chief functions of a telescope, as has already been explained, are the collecting of light, and the magnifying of the image. The first is called the illuminating, and the second the magnifying, power. The illumi- nating power evidently depends on the size of f he object-gla-ss; and supposing that all the light hich fell on it were so refracted as to enter the eye, the illuminating power would be to that of the eye in the proportion of the surface of the object-glass to the surface of the pupil. Thus, if the pupil of the eye be supposed to have a dia- meter of \ inch, and the object-glass a diameter of 6 inches, or a surface 576 times as great as (lie pupil, the image formed by the telescope would be 576 times brighter tlian that formed by the unassisted eye, and the illuminating power would be 676, In general, the ilknninating power is said to be in the proportion of the squares of the diameters. Sir W. Herschel, in his astrono- mical researches, first used telescopes as a means of measuring depth in space. This property of the instrument he named space-penetratinr/ power. By reference to Light, it will be seen that the intensity' of light diminishes as the square of the distance increases; that is, that at twice the dis- tance only \ of the light will fall on any surface, and to render the illumination equal in such a case, the light from four times as much area would require to be collected on the remote sur- face. This would be done by a lens of four times the surface, or of twice the diameter. If an object as seen from the distance 1, in such an instance, had been barely visible, at the distance 2 it would not be seen till the light had been collected by a lens of four times the surface, and when this was done, the illumination would be exactly equal in the two cases. Conversely, if an object, after being removed to a certain distance, where it was barely visible by the unassisted eye, were again to be removed to such a distance that a lens of four times the surface, or twice the diameter of the pupil, held before the eye, again rendered it barely visible, it might be inferred that the dis- tance in the second case was exactly twice that of the first. By similar reasoning, it may be shown that an object-glass of three times the diameter would penetrate into space three times as far, and one of 100 times the diameter would show objects at a distance 100 times as great as the naked eye, or have 100 times as great a space- penetrating power. This determination is founded on the supposition, that the glasses of telescopes transmit to the eye all the light which falls on them. This is never completely realized, even by the most perfect attainable polish and trans- parency, and therefore a small deduction is to be made in the foregoing calculations. Sir W. Her- schel considered that his ten feet telescope had a space-penetrating power of twenty-eight, and that his forty feet instrument could descry an object removed 192 times as far into space, as the utmost reach of human vision. These last-mentioned instruments were reflectors, and their powers are calculated in the same way as the refractors, onlj' that a somewhat greater deduction must be made on account of loss of light in reflection.—The magnifying power of a telescope can easily be deduced from optical principles. The simplest case is that of the astronomical telescope, fig. 5. Fig. ». 82; The apparent magnitude of any object as seen by tiio eye, depends on the size of the visual angle under which it is seen; that is, of the angle contained between the rays from its two