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Science Lectures at South Kensington, by Capt. Abney [and Others].
Science Lectures at South Kensington by Capt Abney - and Others Author:Victoria and Albert museum General Books publication date: 2009 Original publication date: 1878 Notes: This is a black and white OCR reprint of the original. It has no illustrations and there may be typos or missing text. When you buy the General Books edition of this book you get free trial access to Million-Books.com where you can select from more than a million book... more »s for free. Excerpt: MICROSCOPES. BY H. 0. SORBY, F. R. S. Lv tlie course of one hour it is quite impossible to do more than give a very brief outline of the subject of the microscope, for there are a great many departments connected with it, each of which would well deserve to have a special lecture. I might treat this subject on the present occasion in a variety of ways. I feel very much tempted to go into it historically on account of the very magnificent collection of microscopes, of almost all periods, from the very earliest made down to the most recent; but I think it would be more useful to draw your attention to those points which are the most important in practically working with modern instruments. Without further introductory remarks, I will first call attention to the reason of the magnifying power of a single lens. If we have a plano-convex or double convex lens, the rays proceeding from the focus are made parallel by passing through the lens, and as our eyes are only constituted to see distinctly when almost parallel rays enter thom, the result is that if an object be placed in the focus, and the eye be placed on the other side of the lens, an object at that short distance can be seen distinctly. If the lens were not there the object could not be seen distinctly, unless placed at a certain distance, dependent on the length of sight of different individuals. Supposing that the distance at which an individual can see distinctly is ten inches, and that the lens has a focal length of one inch, an object clearly seen in the focus will appear as large as one ten times the diameter at the distance of ordinary vis...« less