OPTICAL INSTRUMENTS. 7 



glass is used, the sines are as 2 to 3 focus, so as to decrease the diameter of 



nearly, and in diamond as 2 to 5. 

 Fig. 11. 



When a ray is passing out of a dense 

 medium into a rarer, the direction will 

 be changed, and the ray bf will now be 

 bent further from the perpendicular, so 

 as to make the sines the reverse of the 

 former case. Out of water they will be 

 as 4 to 3 : from glass as 3 to 2, and 

 from diamond as 5 to 2. The theorems 

 just described for finding the foci of 

 lenses are called geometrical, and will 

 be nearly the same as the refracted, 

 when the lens is made of plate glass. 

 The refracted focus is only oiist.part 

 less than the geometrical, when ascer- 

 tained by accurate experiment. The 

 refracted focus of lenses of other media 

 may be obtained by dividing the geo- 

 metrical focus by the quotient obtained 

 when the sine of incidence (z), minus 

 the sine of refraction (r), is divided by 



half the sine of refraction. ( l \ 

 \ l *r ) 



(12.) Convex Lenses, in their simple 

 state, have been applied to collect the 

 heat of the sun's rays for purposes si- 

 milar to that of burning mirrors. One 

 of the largest lenses that have been 

 mounted for these purposes, was that 

 made of flint glass by Mr. Parker. This 

 lens was 3 feet in diameter, and when 

 mounted, exposed a surface of 330 

 square inches to the sun's light; its 

 focal distance was 3 feet 9 inches, and 

 the diameter of the circular spot of 

 light was one inch. But in order that 

 the light might be condensed as much 

 as possible, he employed another lens, 

 13 inches diameter, and of 29 inches 



the focal point to 3-8ths of an inch. The 

 apparatus on which it was mounted is 

 shown in Jig. 12 : a is the large convex 

 lens mounted in a ring, and connected 

 to the smaller lens b by wooden ribs c c ; 

 the lower rib has a piece e attached to it, 

 capable of adjustment to or from the 

 smaller lens : to this bar is fixed the 

 holder d, having an universal joint. On 

 this holder, the substance to be experi- 

 mented on is placed. The following 

 are some of its effects on bodies placed 

 in its focus ; 20 grains of pure gold was 

 fused in 4 seconds ; 1 grains of pla- 

 tina fused in 3 seconds, and a diamond, 

 weighing 10 grains, exposed for 30 mi- 

 nutes, lost 4 grains. This lens, which 

 is now in the posession of the Emperor 

 of China, cost 7001. 



In large burning lenses the weight of 

 the glass employed becomes of consider- 

 able importance ; and to effect as great 

 a saving as possible, Count Buffon has 

 proposed to construct them of circular 

 rings, as shown in/ig-. 13, where the lens 



Fig. 13. 



is composed of three pieces, two rings, a 

 and b, and a lens c. When, however, 

 the size is very great, the rings may be 



