OPTICS ELECTRICITY. 



97 



fearr., that light flies at the rate of nearly twelve 

 millions of miles every minute that it moves in 

 straight lines that its particles may be several 

 thousands of miles distant from each other that 

 every visible body emits particles of light from 

 its surface, in all directions that the particles of 

 light are exceedingly small ; for a lighted candle 

 will fill a cubical space of two miles every way 

 with its rays, before it has lost the least sensible 

 part of its substance ; and millions of rays, from 

 a thousand objects, will pass through a hole not 

 larger than the point of a needle, and convey to 

 the mind an idea of the form, position, and colour 

 of every individual object that the intensity or 

 degree of light decreases, as the square of the 

 distance from the luminous body increases ; that 

 is, at two yards distance from a candle, we shall 

 have only the fourth part of the light we should 

 have at the distance of one yard ; at three yards 

 distance, the ninth part ; at four yards, the six 

 teenth part, and so on that glass lenses may be 

 ground into the following forms , plano-convex, 

 plano-concave, double convex, double concave, and 

 meniscus, that is, convex on one side, and con 

 cave on the other that specula, or mirrors, may 

 be ground into either a spherical, parabolical, or 

 cylindrical form that, by means of such mirrors 

 and lenses, the rays of light may be so modified 

 as to proceed either in a diverging, converging, 

 or parallel direction, and the images of visible 

 objects represented in a variety of new forms, 

 positions, and magnitudes that every ray of 

 white light may be separated into seven primary 

 colours : red, orange, yellow, green, blue, indigo, 

 and violet that the variegated colouring which 

 appears on the face of nature is not in the objects 

 themselves, but in the light which falls upon 

 them that the rainbow is produced by the re 

 fraction and reflection of the solar rays in the 

 drops of falling rain that the rays of light are 

 refracted, or bent out of their course, when they 

 fall upon glass, water, and other mediums that 

 the light of the sun may be collected into a point 

 or focus, and made to produce a heat more in 

 tense than that of a furnace* that the rays from 

 visible objects, when reflected from a concave 

 mirror, converge to a focus, and paint an image 

 of the objects before it, and that when they pass 

 through a convex glass, they depict an image be 

 hind it. 



* This is produced by means of lenses, or mirrors 

 of a large diameter, called burning-glasses. By these 

 instruments the hardest metals, on which common 

 fires, and even glass-house furnaces, could produce 

 no effect, have been melted in a few seconds M 

 Villette, a Frenchman, nearly a century ago, con 

 structed a mirror, three feet eleven inches in diame 

 ter, and three feet two inches in focal distance, which 

 melted copper ore in eight seconds, iron ore in twenty- 

 four seconds, a fish s tooth in thirty-two seconds, 

 cast iron in sixteen seconds, a silver sixpence in 

 seven seconds, and tin in three seconds. This 

 mirror condensed the solar rays 17,257 times a de 

 gree of heat which is about four hundred and ninety 

 times greater than common fire. Mr. Parker, of 

 London, constructed a lens three feet in diameter, 



On these and other principles demonstrated by 

 this science, the Camera Obscura, the Magic 

 Lantern, the Phantasmagoria, the Kaleidoscope, 

 the Heliostata, the Micrometer Spectacles, 

 Opera-Glasses, Prisms, single, compound, lu 

 cernal, and solar Microscopes, reflecting and re 

 fracting Telescopes, and other optical instru 

 ments, have been constructed by means of whie 

 the natural powers of human vision have bee** 

 wonderfully increased, and our prospects into 

 the works of God extended far beyond what for 

 mer ages could have conceived. 



VI. Electricity. This name has been given 

 to a science which explains and illustrates the 

 operations of a very subtile fluid called the elec 

 tric fluid, which appears to pervade every part 

 of nature, and to be one of the chief agents em 

 ployed in producing many of the phenomena ol 

 the material world. If a piece of amber, seal 

 ing wax, or sulphur, be rubbed with a piece ol 

 flannel, it will acquire the power of attracting 

 small bits of paper, feathers, or other light sub 

 stances. If a tube of glass, two or three feet in 

 length, and an inch or two in diameter, bo rubbed 

 pretty hard, in a dark room, with a piece of dry 

 woollen cloth, besides attracting light substan 

 ces, it will emit flashes of fire, attended with 

 a crackling noise. This luminous matter is 

 called electricity, or the electric fluid. If a large 

 globe or cylinder of glass be turned rapidly 

 round, and made to rub against a cushion, streams 

 and large sparks of bluish flame will be elicited, 

 which will fly round the glass, attract light bo 

 dies, and produce a pungent sensation if the 

 hand be held to it. This glass, with all its re- 



Juisite apparatus, is called an eiectrical machine. 

 t is found, that this fluid will pass along some 

 bodies, and not along others. The bodies ovei 

 which it passes freely are, water, and most othet 

 fluids, except oil and the aerial fluids ; iron, cop 

 per, lead, and in general all the metals, semi- 

 metals, and metallic ores ; which are, therefore, 

 called conductors of electricity. But it will not 

 pass over glass, resin, wax, sulphur, silk, baked 

 woods, or dry woollen substances ; nor through 

 air, except by force, in sparks, to short distances. 

 Thesebodies are, therefore,called non-conductors. 

 The following facts among others, have been 

 ascertained respecting this wonderful agent : 

 That all bodies with which we are acquainted 

 possess a greater or less share of this fluid tha 

 the quantity usually belonging to any body pro 

 duces no sensible effects ; but when any surface 



and six feet eight inches focus, which weighed 212 

 pounds. It melted twenty grains of gokl in four se 

 conds, and ten grains of platina in three seconds. 

 The power of burning glasses is as the area of the 

 lens directly, and the square of the focal distance in 

 versely or, in other words, the broader the mirror 

 or lens and the shorter the focal distance, the more 

 intense is the heat produced by such instruments. 

 A globular decanter of water makes a powerful 

 burning-glass ; and house furniture has been set on 

 fire by incautiously exposing it to the rays of tb- 

 Bun. 



