Metallurgy; an introduction to the study of physical metallurgy . increase, although the amount of hght which istransmitted by the lens is reduced and tbe image thus loses inbrightness. This process of reducing the effective diameter ofthe lens is known as stopping down, and produces two other 48 STUDY OF PHYSICAL METALLURGY important effects besides that of eliminating the aberrationsdue to the outer regions of the lens. The first of these is adecided advantage, while the other entails such serious dis-advantages as to put a practical limit to the application of theprocess. The advantageous e
Metallurgy; an introduction to the study of physical metallurgy . increase, although the amount of hght which istransmitted by the lens is reduced and tbe image thus loses inbrightness. This process of reducing the effective diameter ofthe lens is known as stopping down, and produces two other 48 STUDY OF PHYSICAL METALLURGY important effects besides that of eliminating the aberrationsdue to the outer regions of the lens. The first of these is adecided advantage, while the other entails such serious dis-advantages as to put a practical limit to the application of theprocess. The advantageous effect of stopping down is toincrease the depth of focus and also the flatness of field —^Ln other words, to cause the image to appear sharp over alarger area. The manner in which a reduction of the effective diameter ofthe lens affects depth of focus can best be seen from the dia-gram of Fig. 7, in which the objective is represented by 00;2J is a point in the object from which a bundle of rays, rrr,diverges, passing through the lens, 00, and being again con-. FiG. 7.—Eelation of aperture to depth of focus. verged to the point P in the image. If we think of a secondpoint, s, in the object situated at the same distance from thelens as the first point, p, then S, the image of s, will be situatedat the same distance behind the lens as P, the image of p- Ifthe lens is so placed that the points P and S fall upon thefocussing screen or into the focal plane of the eye-piece, thenthe images represented by these two points wiU appear per-fectly sharp. But now consider a point, g, situated a littlefurther from the lens than either p or s—such a point mightrepresent one of the lower or deeper places in a somewhatdeeply-etched specimen. The image of such a point will lieat Q, nearer the lens than P and S. The result will be that ifthe screen or focal plane is set to suit P and S, that part of theimage representing points like Q will not be sharp—the bundle THE METALLURGICAL MICROSCO
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectmetals, bookyear1922
