. The microscope and its revelations. s various telescopes by simplyexpressing them in inches; but this is obviously not available tothe microscopist, who has to deal with the ratio of two varyingquantities. Professor Abbe here again conferred a boon upon microscopists byhis discovery (in 1873, independently confirmed by Professor Helm-holtz shortly afterwards) that a general relation existed between thepencil admitted into the front of the objective and that emergingfrom the back of the objective, so that the ratio of the semi-diameterof the emergent pencil to the focal length of the objectiv
. The microscope and its revelations. s various telescopes by simplyexpressing them in inches; but this is obviously not available tothe microscopist, who has to deal with the ratio of two varyingquantities. Professor Abbe here again conferred a boon upon microscopists byhis discovery (in 1873, independently confirmed by Professor Helm-holtz shortly afterwards) that a general relation existed between thepencil admitted into the front of the objective and that emergingfrom the back of the objective, so that the ratio of the semi-diameterof the emergent pencil to the focal length of the objective could be FIG. 81.—Relative diameters of the (uti-lised) back lenses of various dry andimmersion objectives of the samepower (i) from an air-angle of 60° toart oil-angle of 180°. 48 VISION WITH THE COMPOUND MICROSCOPE expressed by the sine of half the angle of aperture («) L multipliedby the refractive index of the medium (??.) in front of the objective,or n sin «, (n being IO for air, T33 for water, and 1/5 for oil orbalsam).. FIG. 32.—Illustration of the law of consequence for aplanatic systems. Let 0 and O* (fig. 32) be the conjugate aplanatic foci of a wide-angled system ; u, U the angles of inclination of any two rays admitted 1 In the original translation of the papers of Professor Abbe from German intoEnglish the German mathematical symbols have been retained. In the summary of A of objective-ni sin -573 = -86. oi/ ju^
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectmicrosc, bookyear1901
