24 



OPTICAL INSTRUMENTS. 



175 times, and the highest 700 times: 

 the cost of this instrument was 1300/. 

 sterling. Another telescope has lately 

 been made of similar materials in Eng- 

 land by Mr. Tulley ; the aperture of the 

 object-glass is 6 T 8 ? , and its focal length 

 is 12 feet. This instrument is now in 

 the possession of Dr. Pearson.* 



(38.) The Galilean telescope was in- 

 vented in 1590, by the illustrious person 

 from whom it derived its name ; but as it 

 is susceptible of little improvement from 

 the nature of its construction, it is sel- 

 dom used except for opera glasses, in 

 which the shortness of the construction 

 renders it available. It consists of a sin- 

 gle convex or achromatic object-glass, 

 whose focal length is usually from 4 to 

 8 inches, which it rarely exceeds. The 

 eye-glass is a double or plano-concave 

 lens from an inch to 2 inches focus ; 

 the distance between the two glasses 

 is equal to the difference of their focal 

 lengths, and the power is in the ratio of 

 their foci, as in the astronomical tele- 

 scope. Pig. 26 is a section of the Gali- 



lean construction for an opera-glass. 

 Let O be the object-glass of 6 inches 

 focus, E the concave eye-glass 2 inches 

 focus ; the distance O E will be 4 

 inches, and the power will be expressed 

 by 6 inches divided by 2 inches, equal 

 to 3 times. The distinctness of the Ga- 

 lilean construction exceeds that of any 

 other, and arises from the rays of light 

 proceeding from the object directly 

 through the lenses without crossing or 

 intersecting each other ; whereas, in 

 the combination of convex glasses, they 

 intersect one another to form an image 

 in the focus of the object-^lass ; and 

 this image is magnified by the eye-lens 

 with its imperfections and distortions. 

 With a power of 8 and a 12 or 16 -inch 

 object-glass, the satellites of Jupiter 

 have been distinctly observed ; while a 

 common astronomical telescope of 4 or 

 5 feet focal length has scarcely rendered 

 them visible. The area or field of view 

 in this instrument is very limited, and, 

 on that account, it cannot be used for 

 high powers, as the objects seen at one 



r. 26. 



view are always as the area of the pupil 

 of the eye, and not as the area of the 

 eye-glass, as in convex lenses. Thus, if 

 ab (fig. 20.) is larger than the pupil of 

 the eye they will not be seen, although 

 refracted by the lens E ; but it should 

 be remembered, that as the rays, when 

 they have passed through the eye-glass, 

 are not converged to a focus, the nearer 

 the eye is placed to the" lens E, the more 

 numerous will be the objects seen at one 

 view. 



(39.) The construction of opera- 

 glasses might be achromatic, when 

 made with only two lenses, provided the 

 focal length of the object and eye-lens 

 are in the ratio of the dispersive and 

 refractive powers of the media from 

 which they are formed. Thus, if an 

 object lens be made of rock-crystal, 

 whose focal distance is 5, and the eye- 



* See Astronomical Transactions, vol, ii. 



lens be formed of oil of cassia," fixed 

 between two parallel pieces of glass, or 

 other convenient substance so curved 

 as to give a focus of 1.02, the combina- 

 tion will be achromatic, with an amply- 

 fying power of 4 T 9 o times. If the con- 

 cave lens had been formed of flint-glass, 

 with the same object lens, a magnifying 

 power of two might be obtained, and 

 the combination would be free from all 

 colour. If oil of aniseed were used for 

 the concave, the power obtained to make 

 the instrument achromatic would be 

 2.82 times*. 



The following TABLE exhibits the 

 refractive and dispersive powers of diffe- 

 rent substances capable of producing 

 achromatic combinations ; and it should 

 be remarked, that the medium used for 

 the eye-lens must have the greatest dis- 

 persive power : 



* Dr, firewater's treatise on new optical Instru- 

 cts. 



