1G 



OPTICAL INSTRUMENTS. 



treating on the chromatic correction of 

 lenses. The adjustment of this instru- 

 ment to distinct vision is made by a 

 rack and pinion attached to the sliding 

 piece and great tube of the telescope, by 

 \vhich the eye-piece and small speculum 

 is brought nearer or farther from the 

 large metal. Let r r be the rays of 

 light coming from a distant object, and 

 falling on the large speculum A, these 

 rays would be reflected to the focus e; 

 but meeting with the oblique flat metal 

 c, are reflected to/, where an image of 

 the distant object will be formed, and is 

 received by the eye-lens g, by which the 

 rays are rendered parallel. The power 

 of a Newtonian reflector is proportional 

 to the relative focal distances of the 

 concave metal and the eye-lens. For 

 example, let the focal distance A e be 

 40 inches, and the focus of the eye lens 

 g, half an inch, the power will be 80. It 

 should here be observed, that the same 

 instrument which is free from aberra- 

 tion for astronomical observation will 

 not be so for terrestrial uses ; for the 

 rays in the former case are parallel, 

 while they are divergent in the other. 

 The curve, therefore, of the large spe- 

 culum when required for the latter pur- 

 poses, should be elliptical, having the 



object in one focus, and the focus of 

 the eye-lens in the other. 



This telescope, which is more simple 

 than other reflectors, may be greatly im- 

 proved according to the method of Dr. 

 Brewster, who has proposed, (for tele- 

 scopes of moderate size, where a front 

 view cannot be used,) to employ two 

 glass prisms in place of the small 

 plane. By the experiments of Major 

 Kater, it appears that one-third of 

 the rays of light is lost when re- 

 flected by a speculum at a vertical in- 

 cidence, and probably not more than 68 

 out of 100 are reflected at an angle of 

 45, as in the Newtonian small metal ; 

 in addition to this, the imperfection of 

 surface and figure in metals, which 

 makes the rays stray 5 or 6 times more 

 than the same imperfection in a refract- 

 ing surface,* as well as the difficulty of 

 working metals as perfect as glass, 

 induced him to suggest this improve- 

 ment. Let a b, (Jig. 22,) be the great 

 speculum, and r a, r b parallel rays 

 from a distant object reflected to a 

 focus at F ; the cone of rays, however, 

 is intercepted by the achromatic 

 prism c d, and refracted to /, where a 

 distinct image is formed in the anterior 

 focus of the eye-glass e^by which it is 



Fig. 22. 



magnified. The double prism c d, being 

 composed of a prism of crown glass c, 

 and another of flint d, united by a ce- 

 ment of mean refractive power, the loss 

 of light by transmission through the two 

 prisms, says Dr. Brewster, will not ex- 

 ceed 600 rays out of ] 0,000, as the light 

 transmitted through a lens of glass, ac- 

 cording to Dr. W. Herschell, is 9,485 

 out of 10,000 incident rays. Hence, the 

 light lost by the prism is only of that 

 lost by reflection. 



The Newtonian telescopes made by 

 Hadley had, in place of a plane metal 

 a right angular prism P substituted, 

 haying its sides perpendicular to the 

 incident and emergent rays. In this, as 

 is accomplished by the two prisms of 



Dr. Brewster, the image will be erect, 

 and a less quantity of light lost than 

 by a mirror of the common kind. 



(28.) Another class of reflecting tele- 

 scopes was invented by Dr. Gregory, in 

 1660, but they were not made till some 

 years after the Newtonian, from the dif- 

 ficulty of forming the metals. The Gre- 

 gorian reflector is, however, preferred 

 to the Newtonian, and is most com- 

 monly used, because the observer is 

 stationed in a line with the object, 

 whereas, in the Newtonian he is at 

 right angles to it. Fig. 23 is a sec- 

 tion of the Gregorian reflector. B D 

 is a concave metal, whose surface 



t * jSee Newton's Optics, 



