Ocular refraction and the shadow test . s is true, the effects created are not so noticeable in the weak-er power lenses as m the stronger ones, a lens of two inches so calledfocus has a focal power at the periphery of about inches, whichgives a quite blurred image. The degree of aberration increases withthe size (aperture) of the lens and the power, also the form of thelens and the distance of the object A double convex lens has thegreatest aberration, and gives the most distortion, a piano convexless, and a periscopic the least. Persons who have worn glasses for years of a certain form f
Ocular refraction and the shadow test . s is true, the effects created are not so noticeable in the weak-er power lenses as m the stronger ones, a lens of two inches so calledfocus has a focal power at the periphery of about inches, whichgives a quite blurred image. The degree of aberration increases withthe size (aperture) of the lens and the power, also the form of thelens and the distance of the object A double convex lens has thegreatest aberration, and gives the most distortion, a piano convexless, and a periscopic the least. Persons who have worn glasses for years of a certain form find itdifficult in some instances to use the same power lens but in anotherform; it will be well to remember this fact as it may explain whatmay otherwise seem to be an imaginary grievance of some glasswearer. In cameras and some other optical instruments, the aberra- LENSES. 45 tion is corrected by the use of a diaphragm that cuts out the periph-eral rays and sharpens the image. In the eye this diaphragm effectis supplied by the Figure 45. Distortion of objects created by looliing through strong convex lenses, the square S is dis-torted into the curved sided figure S; etc. Figure 4J shows how objects appear distorted when viewedthrough a strong convex spherical lens, the two vertical parallel linesA and B are greatly magnified when seen through the lens, they are V I rgure 46. A plano-convex; B, concavo-conid D, bi-convex or double convex Types of convex spherical lens. copic; C i no longer straight but appear as two curved lines A B with their con-vex sides toward each other. The square figure S becomes a figure 46 OCULAR REFRACTION. S bounded by four curved sides, their convexity toward the familiar evidence of spherical aberration is seen in photographs ofstreet scenes in which the buildings have the appearance of beingabout to topple into the centre of the street, the effect is created bythe spherical aberration of the lens of the camera. The sections cut f
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