OPTICAL INSTRUMENTS. 



10 



(3 1 .) The largest front view reflecting 

 telescope at present in this country, is 

 that erected at the Royal Observatory, 

 at Greenwich, by Mr. Ramage, in 1820. 

 The diameter of the concave reflector is 

 15 inches, and its focus 25 feet ; the 

 mechanical arrangement of the stand is 

 greatly simplified. A perspective view 

 of the whole instrument is shown at 

 fig. 24. The tube is composed of a 

 twelve-sided prism of deal | inch thick. 

 At the mouth c is a double cylinder of 

 different diameters on the same axis ; 

 around this a cord is wound by a winch, 

 and passes up from the small cylinder 

 over a pulley a, and down through the 

 pulley b, on to the larger cylinder at c. 

 Now, when the winch is turned to raise 

 the telescope, the endless cord is un- 

 wound from the smaller cylinder, and 

 wound on to the larger : the difference of 

 the size of the two cylinders will be 

 double the quantity raised, and a me- 

 chanical force to any extent may thus 

 be obtained by duly proportioning the 

 diameters of the two cylinders ; by 

 this contrivance the necessity for an 

 assistant is superseded. The instru- 

 ment, when not in use, is let down into 

 the box d d, and covered with canvas, 

 to prevent dust or moisture from tar- 

 nishing the speculum. 



CHAPTER VII. Theory of Achromatic 

 Telescopes Double Object- Glass. 



(32.) Having noticed all the valuable 

 modifications of the reflecting telescope, 

 we must now return to the refracting 

 one. The most obvious and important 

 improvement m this instrument consists 

 in the formation of object-glasses free 

 from the errors of chromatic and sphe- 

 rical aberration, whence they were deno- 

 minated achromatic telescopes. But 

 as this word merely signifies freedom 

 from colour, which in common tele- 

 scopes is sometimes effected without a 

 correction of the figure or spherical 

 aberration, Sir W. Herschel has, there- 

 fore, veiy properly denominated a per- 

 fect telescope aplanatic, from two Greek 

 words from xxavos error, that is, free 

 from all errors. 



In (Note to 24) it was stated, that the 

 length of the spectrum produced by 

 lenses varied, when formed of different 

 substances ; thus, if two lenses are 

 made of the same focal length, the one 

 of flint glass and the other of crown, the 



length or diameter of the coloured image 

 in the flint will be to that produced by 

 the crown lens as 3 to 2 nearly. Now, 

 if we make the focal lengths of the lenses 

 in this proportion, that is, as 3 to 2, the 

 coloured spectrum produced by each 

 will be equal ; but if the flint lens be 

 concave and the crown convex, when 

 placed in contact they will mutually 

 correct each other, and a pencil of white 

 light refracted by the compound lens 

 would remain colourless. Unfortunately 

 in the formation of such a lens, the dis- 

 persion of the flint glass is so variable, 

 that trials on each specimen require to 

 be made, before the absolute propor- 

 tional dispersion of the substances can 

 be ascertained*. As the achromatic 

 object-glass is the most delicate test of 

 the dispersion of the medium, it is best 

 found by forming a piece of the flint 

 glass into a concave lens, and combining 

 it with a convex of crown glass whose 

 focal length is known, and varying the 

 curvature of the flint till the dispersions 

 are corrected, i. e. till the purple or lilac 

 fringe surrounding a white object on a 

 black ground is observed on one side of 

 the focus, and a green on the other, 

 when converted into the object-glass of 

 a telescope and using a powerful eye- 

 glass. Now, if the compound focus be 

 accurately measured, and the focus of 

 the convex lens known, the propor 

 tionate foci of each may be ascertained, 

 and, consequently, the proportion of their 

 dispersive powers is found. This cor- 

 rection of the spectrum will not correct 

 the error of each colour, for the propor- 

 tional lengths of the blue, green, or red 

 light are variable in different sub- 

 stances* : thus flint glass is found to re- 

 fract green light considerably less than 

 crown glass, in the proportion of the 

 whole refraction of the red and violet 

 light : so that when the divergency of 



* 1. Sulphuric acid exceeds all other transparent 

 bodies in its action on the green rays, while oil of 

 cassia exerts the least action upon them of all known 

 substances. 



2. It is ascertained that in all minerals in which 

 a metal is the principal ingredient, those which 

 have the greatest density have also the greatest 

 faculty of producing colour, while in all the precious 

 stones a high refractive power is attended with a 

 low dispersive power. 



3. The dispersive powers of resins, gums, oils; and 

 balsams greatly exceed water, and correspond in 

 some measure with their refraction. 



4. The muriatic and nitric acids exceed water in 

 dispersion, while the phosphoric, citric, sulphuric, and 

 tartaric acids, surpassing them in refraction, possess 

 very low dispersive powers. (See the Tables of Re- 

 fractive and Dispersive Powers of different Sub- 

 stances in the Treatise on Optics.) 



c 2 



