We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
24 THE BOOK OF THE LANTERN. L R, after refraction by lens A, diverges into a prismatic beam; this falls upon different parts of lens B, which, while acting upon the two extremes,— the red and the violet,—in contrary directions to A, causes them to emerge parallel, the condition of achromatism, when they converge to the conjugate focus f, about 9 in. removed from B. The spherical aberration is reduced to a minimum by the forms of the lenses employed, i.e., ray Lr, refracted by the central portions of the lenses, meets the axis at the same point f, as the marginal rays, or nearly so. I have decided upon a 4-in. (effective) diameter condenser, since it fully illumi- nates the corners of a 2|-in. square slide. Of course, a circular slide of 3 in. only requires a 3J-in. diameter con- denser, of proportionately shorter focal length. I need hardly say that the glass composing this condenser has been selected with especial care. It is perfectly limpid, or colourless, and will remain so ; it is free from stricB and air bubbles, and has a perfect polish. In fact, it is Chance's best glass ; the only drawback being its cost." Mr. Dallmeyer goes on to remark that the defects in the glass of a lantern condenser, are of far more importance than similar defects in the objective, so far as purity and quality of the illuminated disc are concerned. He refers to such defects as scratches, air-bubbles, and the like. In the case of an air-bubble in the objective lens, it is really of no moment whatever. I have known of a photographic lens, otherwise of splendid quality, being rejected because of a tiny air-bubble near its margin, the purchaser being quite content to exchange it for a far inferior lens without such an insignificant blemish. Such a bubble would have no