. The microscope and its revelations. cannot admit more than the two first diffractionbeams on each side of the axis, the immersion of 133° balsam-angleis capable of admitting from balsam three on each side under exactlythe same It follows, therefoi-e, that a balsam-angle of 75° denotes the sameaperture as the larger air-angle of 133°, and a balsam-angle of 133°a much greater aperture than an air-angle of the same number ofdegrees, and in general two apertures of different objectives must beequal if the sines of the semi-angles are in the inverse ratio of theretractive index of
. The microscope and its revelations. cannot admit more than the two first diffractionbeams on each side of the axis, the immersion of 133° balsam-angleis capable of admitting from balsam three on each side under exactlythe same It follows, therefoi-e, that a balsam-angle of 75° denotes the sameaperture as the larger air-angle of 133°, and a balsam-angle of 133°a much greater aperture than an air-angle of the same number ofdegrees, and in general two apertures of different objectives must beequal if the sines of the semi-angles are in the inverse ratio of theretractive index of the medium to which they relate—or, which is thesame thing, if the product of the refractive index multiplied by sineof the angular semi-aperture (n sin »,) yields the same value forboth, if they are of the same numerical aperture. 2. Suppose the same object to be observed by a dry objectiveof a given air-angle, at first in air uncovered, and then in balsamprotected by a cover-glass. The first case would be represented by. A/Ji sE1^ FIG. 68. fig. 66, and the second by fig. 68. As we have seen, the group ofdiffracted beams from the object in balsam is contracted in com-parison to that in air in the ratio of the refractive index. l>ut 1 The following are the actual angles represented in the diagrams, viz.: (Strise^2-2 ft., wave-length A = 55 /j., medium air » = 1.)SI = 14° 308., = 30° 08^ = 48° 3684=90° (>. rise = 2-2 /j., wave-length A = •.>.> /j., medium balsam «=-r5.)8, =9° 36S2=19°2883 = 30 if 54 = 41° 4* 55 = 56 liC, HOMOGENEOUS VERSUS DRY OBJECTIVES 79 according to the law of refraction, this group, on passing to air l>ythe plane surface of the covering-glass, is spread out—the sines ofthe angles being compared—in the ratio of the same refractive the various diffraction pencils, the first, second. .on every side, after their transmission into air, have exactly thesame obliquity which they have in the case of direct
Size: 2118px × 1180px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, booksubjectmicrosc, bookyear1901
