ON OPTICAL INSTRUMENTS. 331 



quantity of light, this conclusion is not confirmed by experiment : it is 

 probable, however, that the greater heat, produced by a larger lens, is only 

 derived from the greater extent of surface exposed at once to the solar 

 rays. 



Lenses are most commonly made of glass, but sometimes of rock crystal, 

 or of other transparent substances. It is difficult to find glass, especially 

 flint glass, for large lenses, sufficiently free from veins : it has been pro- 

 posed to suffer the melted glass to cool without agitation, and to cut the 

 lens out of any of its strata taken in a horizontal direction ; but this method 

 appears to be liable to several practical objections. Mirrors are made either 

 of glass, coated with an amalgam of mercury and tin, or of metal, as of 

 platina, of silver, or of an alloy of copper and tin, to which a little arsenic 

 and silver are sometimes added. Mirrors of metal are more perfect than 

 those of glass, because they are free from the inconvenience of a double re- 

 flection ; but they are more expensive, and are liable to tarnish. Where a 

 large mirror is required, with a w r eak reflection only, we may employ a 

 single surface of glass, the back of the piece being covered with a black 

 coating of some substance differing little from glass in its refractive density, 

 by means of -which the second reflection is avoided. Dr. Wollaston has 

 very ingeniously applied the effect of the reflection of two plane surfaces, 

 inclined to each other, to the construction of an instrument for drawing, 

 which he calls a camera lucida.* He usually employs the internal re- 

 flection of a prism of glass, of which the four surfaces are ground so as to 

 form proper angles with each other. The image formed by the first sur- 

 face is inverted, and the second reflection restores it to its original posi- 

 tion, but places it in a direction which is at right angles with the direction 

 of the object ; so that when we look down through the prism on a sheet of 

 paper, we see a perfect picture of the objects immediately before us, while 

 at the same time, the aperture through which we look, is only partly occu- 

 pied by the edge of the prism, the remaining part being left open, or simply 

 covered with a lens, for the admission of the direct rays of light by which 

 we may see, at the same time, the paper and the pencil to be employed for 

 making a drawing or a copy of any object placed before us. 



When the image formed by a lens or mirror is received on a smooth but 

 unpolished surface, which is capable of irregular reflection, it is visible in 

 every direction. Such an image is exhibited in the camera obscura, the 

 solar microscope, and the magic lantern, or lucernal microscope. 



The general effect of the camera obscura f is the same as may often be 

 observed in a dark room, where there is a small hole in the window shutter : 

 the great masses of light and shade, before the windows, being represented 

 in an inverted position, in the parts of the room diametrically opposite to 

 them, which are illuminated in different degrees, according to the quantity 



* Nich. Jour. 8vo, xvii. 1. Compare Wren, Ph. Tr. 1669, iv. 898. Peacock, 

 ibid. Ixxv. 366. Ramsden and Jones, Phil. Jour, xxviii. Brewster's Account of 

 New Phil. Insts. An account of the modifications which Prof. Amici has effected in 

 \his instrument is given in the Edin. Jour, of Sci. v. 157. Chevalier, Notice sur 

 1'Usage des Chambres Obscures et des Chambres Claires, Par. 1829. Ludicke, 

 Gilb. Ann. xlii. 338. 



f Invented by Baptista Porta, Magia Naturalis, p. 12, Lug. Bat. 1650. 



