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. 1888 Excerpt: ...the object at which we look. Indeed, should we view an object through an instrument which magnified but did not increase the amount of light received by the eye, it is evident that the brilliancy would be diminished in proportion as the surface of the object was enlarged, since a constant amount of light would be ...
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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. 1888 Excerpt: ...the object at which we look. Indeed, should we view an object through an instrument which magnified but did not increase the amount of light received by the eye, it is evident that the brilliancy would be diminished in proportion as the surface of the object was enlarged, since a constant amount of light would be spread over an increased surface; and thus, unless the light were brilliant, the object might become so darkened as to be less plainly seen than with the naked eye. How the telescope increases the quantity of light will be seen by considering that when the unaided eye looks at any object, the retina can only receive as many rays as fall upon the pupil of the eye. By the use of the telescope as many rays can be brought to the retina as fall on the entire object-glass. The pupil of the human eye, in its normal state, has a diameter of about one-fifth of an inch; and by the use of the telescope the retina is virtually increased in surface in the ratio of the square of the diameter of the objective to the square of one-fifth of an inch. Thus, with a two-inch aperture to our telescope, the number of rays collected is one hundred times as great as the number collected with the naked eye. THE POWER OF THE EYE AND OF THE TELESCOPE CONTRASTED. If the brightness of a star seen with the eye alone is 1, with a 2inch telescope it is 100 times as bright, with a 4-inch telescope it is 400 times as bright, 8-inch telescope it is 1,600 times as bright, 16-inch telescope it is 6,400 times as bright, 32-inch telescope it 1s 25,600 times as bright, 36-inch telescope it is 32,400 times as bright. That is, stars can be seen with the 36-inch telescope which are 30,000 times fainter than the faintest stars visible to the naked eye. While the magnifying power which can bo ...
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