CERAUNICS. 361 



We must not omit to notice the mechanism of vision. The outer 

 coating of the eye is transparent in front ; and this part is called the 

 cornea: while the schrotica, which covers the rest of the eye, is 

 opaque and white, but lined with the choroid coat and a black pig- 

 ment ; within which, is the retina, a nervous coating, connected 

 with the brain. The iris, is a dark screen, behind the cornea ; from 

 which the eye takes its colour : and the pupil, is a circular aperture, 

 or window, through the iris, by which light is admitted to the dark 

 chamber within. In this chamber is the crystalline lens, held in its 

 place by the ciliary processes ; with the aqueous humor in front of 

 it, and the vitreous humor in the back part of the eye. All the rays 

 coming from any one point of a luminous object, and entering the 

 pupil, are refracted, by the interior media, to one point or focus on the 

 retina, by means of which the luminous point is perceived. The 

 same principle applies to the camera obscura; in which all the rays 

 from any one external point, are reflected and refracted, so as to form 

 one point of the image or picture within. In the magic lantern, 

 the image formed by refraction, is enlarged, and received on a screen, 

 which corresponds to the retina of the eye. Of telescopes and 

 microscopes, referred to in the preceding divisions, we have no 

 farther room to speak in this brief synopsis. 



CHAPTER IV. 



CERAUNICS. 



WE would propose the name of Ceraunics, for that branch of 

 Natural Philosophy, which relates to the effects of heat and electri- 

 city ; commonly associated in nature, and together forming one great 

 division of science.* The name is from the Greek, sstpawos, light- 

 ning ; a phenomenon in which the union of heat and electricity is 

 peculiarly manifested ; and the study of which involves that of nearly 

 all the sub-branches here included ; that is to say Calorics, Electricity, 

 Galvanism, Magnetism, Electro-Magnetism, Thermo-Electricity, and 

 Meteorology. The order in which these branches are here named, is 

 that in which we think they may best be understood : as those first 

 named, can be studied independently; but the others, only by their 

 aid. They all belong strictly to Experimental Philosophy ; and they 

 have already contributed largely to the comfort of mankind, by their 

 applications to the Physical Arts : but some of these sciences are still 

 in comparative infancy, and open a wide field for new researches. 



Following our general order, the history of these sciences will first 

 engage our attention. The study of Calorics, as a connected science, 

 is of modern origin. Lord Bacon conceived heat to depend upon a 

 vibration of the particles of matter, tending to separate them ; an idea 



* Should it appear hereafter, as may be the case, that heat consists in undulations 

 of an ethereal medium, like those of light ; it will then be proper to remove Calorics 

 from this branch, and associate it with Optics ; with which it has close relations, 

 already recognized. 



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