. Light, a textbook for students who have had one year of physics. The bestknown of these is the Ramsden eyepiece, which functions verysatisfactorily, and is used on most telescopes. It is shown infigure 55. The objective of the telescope has been omitted from the drawing, but the realimage ab and cones of lightforming its ends are shown justas they are in figure 54. Theeyepiece consists of two identi-cal planoconvex lenses, mountedrigidly in a metal tube, and separated by a distance ecpnal to-;.. the focal length of either. It can be shown that such a pairis equivalent, so far as magnificatio
. Light, a textbook for students who have had one year of physics. The bestknown of these is the Ramsden eyepiece, which functions verysatisfactorily, and is used on most telescopes. It is shown infigure 55. The objective of the telescope has been omitted from the drawing, but the realimage ab and cones of lightforming its ends are shown justas they are in figure 54. Theeyepiece consists of two identi-cal planoconvex lenses, mountedrigidly in a metal tube, and separated by a distance ecpnal to-;.. the focal length of either. It can be shown that such a pairis equivalent, so far as magnification is concerned, to a singlelens whose focal length is % that of either component. Thefront Lens, called the field-lens, is placed very close to the realimage ab. and therefore forms from it a virtual image ab,slightly Larger, and slightly farther away. In fact, ab, comesjust at the principal focus of the rear lens of the combination(called the eye-lens) or just within it. Accordingly, this latterforms the final virtual image, AB of figure 54, either at infinity. OPERA GLASS 101 or at whatever distance from the eye is most suitable for theobserver, adjustment being secured by sliding the whole eye-piece toward or away from the real image ab. Since ab liesclose to the field lens, the latter receives and transmits lightfrom an area of the image practically equal to the area of thelens itself, thus giving a field whose diameter is approximatelyequal to the diameter of the field-lens. The pencils from allparts of the image cross the axis just behind the eye-lens. Thisis the most convenient place for the exit-pupil, for the eye canthen be placed close up to the end of the eyepiece. It is foundfurther that this combination of two planoconvex lenses isalmost free from spherical aberration, and the chromatic aber-ration is of such a nature asi to be hardly perceptible. If a micrometer is used with the telescope, it is placed sothat the crosshairs move exactly in the plane ab in which th
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectlight, bookyear1921
