HEESCHEL. 205 



eat, but receives from the bands of bis sister that nourish id eut witliout 

 which one coukl not undergo such prolonged fatigue. Nothing coukl 

 induce Herschel to leave his work ; for, according to him, if he did so 

 it would be to spoil it." 



The advantages that Herschel found in 1783, 1784, and 1785, in em- 

 ploying telescopes of tv/enty feet and with large apertures, made him 

 desire to construct one much larger still. The expense would be consid- 

 erable ; the King provided for it. The work, begun about the close of 

 1785, was finished in August, 1789. This instrument had an iron cylin- 

 drical tube, thirty-nine feet four inches in length, and four feet ten 

 inches in diameter. Such dimensions are enormous as compared with 

 those of telescopes previously made. They will appear but small, how- 

 ever, to persons who have heard the report of a pretended ball given in 

 the Slough telescope. The propagators of this j^opular rumor must 

 have confounded the astronomer Herschel with the brewer Mens, and a 

 cylinder in which a man of the smallest stature could scarcely stand 

 upright, with divers wooden vats, as large as a house, in wliich beer 

 is made in London, 



Herschel's telescope, forty English feet* in length, allowed the reali- 

 zation of an idea, the advantages of which would not be sufficiently 

 appreciated if I did not here recall to mind some facts. 



In any telescope, whether refracting or reflecting, there are two prin- 

 cipal parts : the part that forms the aerial images of the distant objects, 

 and the small lens by the aid of which these images are magnified just 

 as if they consisted of radiating matter. When the image is produced 

 by means of a lenticular glass, the place it occupies will be found in the 

 prolongation of the line that extends from the object to the center of 

 the lens. The astronomer, furnished with an eye-piece and vtishing to 

 examine that image, must necessarily place himself heyond the point 

 where the rays that form it have crossed each other ; heyond, let us caref ally 

 remark, means farther q^' from the object-glass. The observer's head, 

 cannot then interfere with the formation or the brightness of the image, 

 however small may be the distance from which he may have to study it. 

 But it is no longer thus with the image formed by means of reflection. 

 For the image is now i)laced between the object and the reflecting mir- 

 ror, and when the astronomer approaches in order to examine it, he 

 inevitably intercepts, if not the totality, at least a very considerable 

 portion, of the luminous rays, which would otherwise have contributed 

 to give it great distinctness. It will now be understood why, in optical 

 instruments where the images of distant objects are formed by the re- 

 flection of light, it has been necessary to carry the images, by the aid 

 of a second reflection, out of the tube that contains and sustains the 



* Couformiog to general usage, and to Sir W. Herschel himself, we shall allude to 

 this instrument as the forty-foot telescope, though M. Arago adheres to thirty-nine feet 

 and drops the inches, probably because the Parisian foot is rather longer than the 

 English. — Translator's Note. 



