MICROSCOPES. 



BY H. C. SORBY, F.R.S. 



IN- tlie course of one hour it is quite impossible to do more 

 than give a very brief outline of the subject of the micro- 

 scope, 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 them, 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 vision, because it will subtend as 



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