OPTICAL INSTRUMENTS. 



15 



circle of dispersion ^th of an inch in 

 diameter, it may be surprising that any 

 picture of an object can be distinguished ; 

 but the superior vivacity of the orange 

 and yellow light in comparison with the 

 rest, make the effect produced by the 

 confusion of the colours much less sen- 

 sible, and will allow this aperture to be 

 used when the focal length of the lens 

 is considerable. 



(25.) We may now compare the diame- 

 ters of the circles of the chromatic and 

 spherical aberration together. If we take 

 a standard telescope of approved good- 

 ness, it has not been found possible to 

 give more than 4 inches aperture to an 

 object-glass of 100 feet focal distance, 

 so as to preserve sufficient distinctness ; 

 and if the diameter of the circle of sphe- 

 rical aberration is computed for this lens, 

 it will not exceed iso'oooth part of an 

 inch, while the chromatic, if restricted 

 to 3 ioth of the aperture, which is hardly 

 a fifth of the whole dispersion, (or dia- 

 meter of the circle of orange and yellow 



light,) is ggi of an inch, and is there- 

 fore about 1900 times greater than the 

 other. But when the aperture of a lens 

 is increased to 30, the spherical aberra- 

 tion will be found equal to the chroma- 

 tic in a glass lens ; but this aperture can 

 only be used for eye lenses or micro- 

 scopes. 



CHAPTER VI. Reflecting Telescopes 

 the Newtonian the Gregorian the 

 Cassegrainian Sir W. Herschell's 

 Mr. Ramage's. 



(26.) With these disadvantages to con- 

 tend against in refracting substances, 

 Sir Isaac Newton, in the year 1666, 

 turned his attention to reflected light, 



in which the angle of all the coloured 

 rays are equal. By pursuing this idea he 

 entirely obviated the chromatic error. In 

 the first telescope he made by reflection, 

 the distinctness with which objects were 

 seen through it was surprising, when 

 compared with the refracting telescopes 

 of those times ; for though the focal dis- 

 tance of the metal was only 6 inches, 

 it would carry a power of 38 with equal 

 distinctness to a 4 feet refractor. The 

 form of the metal was spherically con- 

 cave ; but by investigation he ascer- 

 tained that if the form had been that of 

 a parabola, there would not have been 

 any spherical aberration produced ; and 

 if we examine the spherical aberration 

 by figure of a spherically concave metal, 

 and compare it with that of a plano- 

 convex lens ground in the same tool, 

 the former will be 4, while the latter 

 is 9. But when it is considered that the 

 focus of the glass lens is 4 times that 

 of the metal, (for the focal distance of a 

 plano-convex lens is twice the radius, 

 and that of a concave reflector half the 

 radius,) to make their foci equal, the 

 curvature of the lens must be 4 times 

 that of the speculum ; and it has been 

 shown that the error by figure increases 

 inversely as the square of the radius : 

 hence the aberration of the lens will be 

 to that of the reflector as 4 a x 9 to 4, 

 or as 36 to 1, and the distinctness will 

 be inversely as the areas of these circles, 

 which are as the squares of their respec- 

 tive diameters ; so that the distinctness 

 of a reflector will be 1296 times greater 

 than that of a lens of the same focus and 

 aperture. 



(27.) The Newtonian telescope (fig. 

 21.) consists of a concave parabolic 

 metal A, fixed at the end of the tube 

 ddd; the plane speculum c is fixed to 



(Fig. 18.) 



a wire, having its other end attached to 

 a dove-tailed sliding-piece i i, and the 

 face of the plane metal is inclined to the 

 axis of the tube and the large speculum 

 at an angle of 45. In the sliding piece 

 i i, opposite the small metal, is inserted 

 a short tube to hold an eye-piece, which 



is a single lens with its flat side outer- 

 most, or the astronomical eye-piece 

 (fig. 19.) ; but as the colour produced 

 by these eye-lenses is not corrected, 

 another combination, called the nega- 

 tive achromatic eye-piece, should be 

 used, which will be described when 



