10 



OPTICAL INSTRUMENTS, 

 Fig. 15. 



the angle c e d, which is four times the 

 angle cod that the object subtends, for 

 the distance Fo is four times Fe : hence 

 the magnifying power may be found, by 

 dividing the focal length of the object- 

 glass by the focus of the eye-glass, when 

 the quotient will be the power. Objects 

 seen through this telescope are in- 

 verted, and on that account it is inap- 

 plicable to land observation ; but at sea 

 it is occasionally used at night, and in 

 hazy weather when there is little light ; 

 it is hence called a night telescope. 



(16.) The common astronomical tele- 

 scope is of the same principle of con- 

 struction as the preceding. The inver- 

 sion of the object is immaterial in its 

 application to celestial observations ; 

 but the disadvantage of this instru- 

 ment is felt when very high powers are 

 required, for then the objects are ren- 

 dered dark and obscure, and if the 

 aperture of the object-glass is increased 

 to admit more light, the formation of 

 the object is confused. M. Huygens, 

 however, made a telescope of this con- 

 struction, in which he was enabled to 

 use an aperture of 6 inches, by making 

 the focus of the object-glass 123 feet in 

 length, and, by changing the eye-lenses, 

 any required power was produced. From 

 experiments on different combinations, 

 he found that to obtain the greatest 

 distinctness and light, the focus of the 

 object-glass, its aperture, and the power 

 of the instrument, should be according 

 to the following table : 



Focus of 

 the Kye 

 Glass. 

 Inches. 



0.605 



0.84 



1.04 



1.18 



1.33 



1.88 



2.68 



3.28 



3.76 



4.20 



5.95 



6.52 



Magnifying 1 

 Power. 



20 



27.6 



33.5 



39.5 



44 



62 



88 

 108 

 125 

 140 

 197 

 216 



(17.) The common day -telescope is an 

 instrument of this class, with the addi- 

 tion of two other lenses of the same 

 power as the eye-lens e ; these lenses 

 will produce an erect image of the ob- 

 ject when placed at a fixed distance 

 from each other, equal to the sum of 

 their two focal lengths. Let o (fig. 1 6.) be 



Fig. 16. 



