72 



ON THE GENERAL DIFFUSION OF KNOWLEDGE. 



rigln or left, for throwing light upon the object 

 placed over the hole i, which may be laid upon a 

 slip of thin glass. The object-glass AB, fig. 2, is 

 placed at u, tig. 3. The glass CD is placed oppo 

 site W, fig. 3, and the eye-glass EF opposite V. 

 Such are the essential parts of a compound 

 microscope. Any common mechanic may con 

 struct one for himself by attending to the follow 

 ing directions : The object-glass AB, fig. 2, 

 may be about , |, or 1 inch focal distance, and 

 the aperture, or hole which lets in the light from 

 the object, should not exceed 1-1 Oth of an inch, 

 otherwise it will cause a glare, which will pro 

 duce an indistinct image of the object. The 

 amplifying glass CD may be 2 inches focal 

 distance, and 1$ inch in diameter. This glass 

 is not essentially necessary, but it serves to 

 enlarge the field of view, and to render it more 

 distinct near the border. The eye-glass EF 

 should be about 1 inch focus, and about inch in 

 diameter. With respect to the distances at 

 which they should be placed from each other, 

 the glass CD may be placed at about 5 or 6 

 inches from AB, and the glass EF about 2 

 inches, or If inch from CD. The object-glass 

 shoule be a double convex the eye-glasses may 

 be plano-convex ; that is, plane on the one side 

 and convex on the other, with the plane sides 

 turned next the eye ; but double convexes will 

 do, if these cannot be procured. The tubes 

 which contain the glasses may be made of 

 pasteboard, and the stage, pillars, and box, of 

 wood. The glasses may be procured for about 

 4 shillings ; and if the individual fit them into the 

 tubes, and perform all the other operations re- 

 q-iisite, the expense of all the other materials 

 will not exceed other four shillings. Suppose, 

 now, that the object-glass AB is inch focal 

 distance, and the image GH is formed at the 

 distance of 6 inches from it, this image will be 

 larger than the object, nearly in the proportion 

 of 6 to |, or 12 times. Suppose the glass EF, 

 considered in connexion with CD, to possess a 

 magnifying power equal to 5 times, then the 

 whole magnifying power will be 5 x 12, or 60 

 times. The object, therefore, will be magnified 

 60 times in length and in breadth, and, conse 

 quently, the surface will be magnified 3600 

 times, which is the square of 60. With such a 

 microscope, the animalcula3 in water, the circu 

 lation of the blood in frogs and fishes, the small 

 feathers which compose the dust on butterflies 

 wings, and all the most interesting appearances 

 of the minute parts of animals and vegetables, 

 maybe distinctly perceived. 



Besides the discoveries in the heavens and in 

 the minute parts of creation, to which the study 

 of the science of optics has led, its principles 

 we capable of being directed to many important 

 purposes in *&amp;gt;uman life and society. By means 

 of large burning mirrors and lenses the rays of 

 the sun have been condensed, so as to increase 



their intensity more than seventeen thousand 

 times, and to produce a heat more than four 

 hundred times greater than that of our common 

 fires, which would serve for the combustion and 

 fusion of numerous substances, which are infu 

 sible in the greatest heat oiat can be produced 

 in our common furnaces. The property of a 

 convex lens, by which rays proceeding from its 

 focus are refracted into parallel directions, has 

 enabled us to throw, from light-houses, a strong 

 light to great distances at sea. The large 

 polyzonal, or built up lenses, con! rived by Sir 

 D. Brevvster, which may be made of any mag 

 nitude, and the elegant lamp of Lieutenant 

 Drummond, the one producing the most intense 

 light yet known, and the other conveying it 

 undispersed to great distances, promise tc 

 introduce improvements hitherto unthought of, 

 and to diversify the nocturnal scenery both of 

 sea and land. For, in the progress of extensive 

 national improvements, they might be made 

 subservient, in connexion with curburetted hy 

 drogen gas, in enlivening and decorating iho 

 rural scene, in the absence of the sun, and in 

 guiding the benighted traveller in all his jour- 

 neyings. For, when we consider the improve 

 ments, in almost every department of the social 

 state, which have been lately carried forward, it 

 is surely not too much to expect, that, in the 

 course of a century hence, our highways, vil 

 lages, hamlets, and even some of our moors and 

 mountains, shall be lighted up witli gas lamps, 

 connected with mirrors and lenses, analogous to 

 those which illuminate our cities and towns, and 

 which direct the mariner, when approaching our 

 shores. The following figure shows the manner 

 in which a large lens throws a light to great dis 

 tances. Let AB, fig. 4, represent one of Sir D. 



Fig. 4. 

 A 



Brewster s polyzonal lenses, or any other largo 

 lens, and GK its focal distance ; if a luminous 

 body CK, as the flame of a lamp, be placed at 

 the focal point K, the rays of light, diverging 

 from CK, after passing through the lens AB, 

 will proceed in a parallel direction, AE, GH, 

 BF, and may illuminate objects at very consi 

 derable distances. AB, fig. 4, represents a 



