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. 1892 Excerpt: ...distance from the lens to the negative, 1'4." Let us check this by calculation. The major conjugate = (n + 1) f (1-65 + 1)10" 26"'5 2'-2j" major conjugate minor conjugate =-----n 26-5.. =--z in inches. 1-65 16"-1 1'-4"i The accuracy of result must, of course, depend on the care in the manufacture of the instrument. It ...
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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. 1892 Excerpt: ...distance from the lens to the negative, 1'4." Let us check this by calculation. The major conjugate = (n + 1) f (1-65 + 1)10" 26"'5 2'-2j" major conjugate minor conjugate =-----n 26-5.. =--z in inches. 1-65 16"-1 1'-4"i The accuracy of result must, of course, depend on the care in the manufacture of the instrument. It is, perhaps, hardly necessary to point out that in the case of reduction the figures remain the same, but the major axis is now the distance of the negative from the lens, and the minor axis the distance of the lens from the reduction. CHAPTER XXI. DEPTH OF FOCUS. A Paradox.--In discussing this subject, we begin somewhat paradoxicallyYby stating that there is no such thing as depth of focus. Optically speaking, the focus of a lens is a point; and in cases where, from aberrations, the rays from any object do not converge to a point, of such a lens it may then be said that it possesses no true focus at all. But, it may be asked, How does it happen that if an object at a reasonable distance--say, a quarter of a mile--be sharply focussed, all objects beyond that will also be sharp? To meet this we say that if a lens of long focus capable of yielding a sharp image be employed, this will not be found to be the case. If the focus of an object at the distance of a mile be carefully found in a telescope of, say, eight inches aperture and proportionate focus, another object situated a quarter of a mile away from the former will be quite out of focus. Brewster, in his Treatise on New Philosophical Instruments, shows that he was quite aware of this property in lenses, for he gives instructions how, by means of a graduated eye-tube, a telescope may be 124 NATURE OF DEFINITION. constructed which shall, within certain limi...
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