The Achromatic Telescope. 453 



excepting to advantage, even beyond their real deserts, after 

 the cumbrousness and unwieldiness of the old refractors in 

 whose place they succeeded ; and those which are now in turn 

 replacing" them are certainly often of previously unattained 

 excellence. 



The great extension, however, of the means of observation 

 has probably not been accompanied with an equal amount of 

 information as to their nature ; and many an amateur may be 

 working with a tool, the principle of which he would find 

 interesting if he did but understand it. Such information is 

 certainly not essential to success, but as it involves some 

 remarkable optical facts, our readers will perhaps not be dis- 

 pleased with a simple account of the construction of the ordi- 

 nary achromatic telescope, as well as of its various modifications. 



As we are not going to write a treatise upon Optics, some 

 obvious preliminaries may be taken for granted ; and we shall 

 consider it to be generally understood that lenses of glass, or 

 other transparent material denser than air, if their curves are 

 predominantly convex, cause the rays of light to converge to a 

 focus, where they form a picture of any object from which they 

 have issued. A spectacle lens for an aged sight placed against 

 the keyhole of a door of a darkened room, with a piece of white 

 paper in its focus, gives a pleasing illustration of this, and every 

 burning-glass is an instance of it, the little fiery spot being a 

 picture of the sun, frequently attended by that of the bright 

 clouds immediately around it. This focal image does not re- 

 quire a screen of paper to render it visible if the eye is placed 

 at a suitable distance behind it, so as to receive the rays after 

 they have crossed at the focus : it will then appear as a picture 

 of the object beyond the lens, always inverted and enlarged, if 

 the focus is sufficiently long; but, as the magnifying power 

 thus obtained is inconsiderable, it is necessary to interpose 

 between the picture and the eye a lens of short focus, or com- 

 bination of lenses, which, acting precisely the same part as a 

 microscope, will form a greatly enlarged image on the retina, 

 and in so doing produce the simplest kind of telescope.* It is 

 evident, therefore, that the quality of such a telescope, and, in 

 fact, of all telescopes, will depend upon the accuracy of the 

 focal image, every defect there being absolutely irremediable 

 afterwards, and only made more conspicuous by each increase 

 of magnifying ; and accordingly the first object of the optician 

 is to obtain as great perfection as possible in the formation of 

 the picture in the focus. To produce this there must be good 

 material, not only as transparent and colourless as may be, but 

 (what is much more important) of uniform density throughout ; 



* This is not, however, the Galilean construction ; which, though equally 

 imple, does not admit of an equally ready explanation. 



VOL. IV. — MO. VI. H H 



