1 84 SCIENTIFIC APPARATUS. 



Rosse, and by the opticians of our own time. Hall and Dollond, 

 however, concurrently with the improvement of the reflecting 

 telescope, showed that Newton's dictum with regard to the refract- 

 ing one was unfounded, and now the refracting telescope is made 

 as compact, if not indeed more compact than the reflecting one, 

 and with every improvement in the manufacture of glass, the size 

 of the object-glass has been increased ; so that now the half-inch 

 aperture of Galileo is represented by the 25-inch aperture of the 

 telescope belonging to Mr. Newall, made by Cooke of York ; by 

 the 26-inch apertures of the telescopes recently finished in the 

 United States, by Alvan Clarke of Boston; and by the 27-inch 

 aperture which is now being constructed for the Austrian Govern- 

 ment by Mr. Grubb of Dublin, a model of which is exhibited. 



Even now, however, the reflector still holds its own in point of 

 size. The 2-inch metallic speculum of Newton was extended to 

 four feet by Sir William Herschel, to six feet by Lord Rosse ; to 

 four feet again by Mr. Lassell, and after the introduction of a new 

 chemical process, by which a film of silver of the utmost brilliancy 

 could be deposited on a surface of glass, the heavy metallic 

 speculum, sometimes weighing tons, has now given way to a 

 much lighter and thinner one of glass, which has the distinct 

 advantage of keeping its figure for any length of time, so that now 

 the modern reflecting telescope is perhaps most adequately repre- 

 sented by the 4-foot silver-on-glass speculum of the magnificent 

 equatorial reflector recently erected in the Observatory at Paris. 



So much, then, for the means of collecting light. If it is simply 

 intended to use this greater quantity of light for the purpose of 

 increasing the power of the eye various eye-pieces are used, the 

 construction of which depends upon principles which have been 

 introduced from time to time by Newton, Herschel, Ramsden, 

 Airy, and others. The most interesting applications of the large 

 telescope in our present Observatories are those connected with 

 other uses. Those, namely, which deal with the spectroscopic 

 examination, or the polariscopic examination of light, or, again, 



