ON OPTICAL INSTRUMENTS. 423 



it is usual ot employ the minutest lenses that can be ground, and sometimes 

 a small globule is formed by fusion in a lamp. Even a drop of water, placed 

 in the perforation of a plate, makes a tolerable magnifier; and it has been pro- 

 posed to substitute for water a transparent varnish, which is less liable t^ 

 evaporate. • 



Supposing the whole light that proceeds from a distant object, and falls on g, 

 lens or speculum, to be collected in the image, its intensity must be increased 

 in the ratio of the surfaceof the lens or speculum to that of tile image. The image 

 is greater in proportion as the object is greater; consecpiently the deoree of 

 condensation produced by any lens is the greater as the object is smaller, thus 

 if the diameter of a lens were an inch, and the image of the sun formed by it 

 were also an inch in diameter, the density of the light would be unaltered; 

 but the image of a star would be infinitely brighter than the direct light of 

 the star falling on the lens. The illumination of any image formed by a 

 lens or mirror, supposing no light to be lost, is always the same as would 

 be produced by the direct light of the surface of the lens or mirror, if it were 

 equally luminous with the surface of the object which emits the li<>-ht. It 

 may also be shown, that 'when two lenses are of similar forms, their focal 

 lengths being proportional to their diameters, they must produce the same 

 degree of illumination in the image: but as far as the heat excited may be 

 supposed to be a measure of the 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 proposed 

 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 anil' 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 reflection ; but thev 



