The microscope and its revelations . Formation of Images by convex lenses. the lens, the image a b will be nearer to it, and smaller thanitself. Further, it is to be remarked, that the larger theimage in proportion to the object, the less bright it wiU be,because the same amount of light has to be spread over a SPHEEICAL ABEE-EATION. 45 greater surface; whilst an image that is smaller than theobject, wiU be more brilliant in the same proportion. 9. The knowledge of these general facts wiU enable nsreadily to understand the ordinary operation of the J\iicro-scope; but the instrument is subject
The microscope and its revelations . Formation of Images by convex lenses. the lens, the image a b will be nearer to it, and smaller thanitself. Further, it is to be remarked, that the larger theimage in proportion to the object, the less bright it wiU be,because the same amount of light has to be spread over a SPHEEICAL ABEE-EATION. 45 greater surface; whilst an image that is smaller than theobject, wiU be more brilliant in the same proportion. 9. The knowledge of these general facts wiU enable nsreadily to understand the ordinary operation of the J\iicro-scope; but the instrument is subject to certain optical imper-fections, the mode of remedying which cannot be compre-hended without an acquaintance with their nature. One ofthese imperfections results from the unequal refraction of therays which have passed through- lenses, whose curvatures areequal over their whole surfaces. If the course of the rayspassing through an ordinary convex lens be carefully laid-down (Fig. 6), it will be found that they do not all meet. Diagram illustrating Spherical Aberration. exactly in the foci already stated, but that the focus f of therays ab, ab, which have passed through the peripheral portionof the lens, is much closer to it than that of the rays ab, ab,which are nearer the line of its axis ; so that, if a screen beheld in the former, the rays which have passed through thecentral portion of the lens will be stopped by it before theyhave come to a focus; and if the screen be carried back intothe focus of the latter, the rays which were most distant fromthe axis wiU have previously met and crossed, so that theywill come to it in a state of divergence, and will pass toc and d. In either case, therefore, the image will have acertain degree of indistinctness; and there is no one point towhich aU the rays can be brought by a single lens of sphericalcurvature. The difference between the focal points of thecentral and of the peripheral rays, is termed the SphericalAberration. It is ob
Size: 2116px × 1181px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, booksubjectmicroscopes, booksubjectmicroscopy
