A text-book of physics, largely experimentalOn the Harvard college "Descriptive list of elementary physical experiments." . tely for the manufac-turers and users of optical instruments, it is possible tomake an achromatic lens; that is, a lens nearlyfree from chromatic aberration. This is usually\B accomplished by uniting into one lens two sepa-rate lenses,* one convex, or converging(§ 159), of crown-glass, the other concave,or diverging/ of flint-glass, like the B and A ofFig. 142, the whole making a convex combina-tion. In such a compound lens the part A hasa tendency to undo, or counteract,
A text-book of physics, largely experimentalOn the Harvard college "Descriptive list of elementary physical experiments." . tely for the manufac-turers and users of optical instruments, it is possible tomake an achromatic lens; that is, a lens nearlyfree from chromatic aberration. This is usually\B accomplished by uniting into one lens two sepa-rate lenses,* one convex, or converging(§ 159), of crown-glass, the other concave,or diverging/ of flint-glass, like the B and A ofFig. 142, the whole making a convex combina-tion. In such a compound lens the part A hasa tendency to undo, or counteract, the work FIG. 142. of B The superior dispersive power (§ 157) offlint-glass enables A, though of less general refractivepower than 5, to counteract the dispersive tendency ofthe latter, while leaving to the combination as a whole aconsiderable refracting and converging power. Many of the lenses used in optical instruments of thebest quality are achromatic. Eye-pieces (§ 187), however,of the ordinary pattern do not require achromatic lenses. * Sometimes more than two pieces are employed in making anachromatic REFRACTION OF LIGHT. 199 A large lens practically free from spherical and chromaticaberration is a marvel of skilful and patient work. Glasssuitable for making a large lens of the best quality is verydifficult to procure, as a very slight flaw or unevenness ofquality may spoil a large block. The shaping and polish-ing and testing of the largest lenses, after the proper kindof glass is obtained, is a work of years, and men who areskilful and patient enough to do it become known through-out the world. See § 189. QUESTIONS AND PROBLEMS. (1) An object is placed at a great distance from a converging lensand on its principal axis. (a) What changes of position will the image of this object undergowhile the object is moved along the principal axis up to the surfaceof the lens ? (6) In what part of this operation will the image be erect and inwhat part inverted ? (c) In
Size: 945px × 2645px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, bookpublishe, booksubjectphysics
