denominated the Aerial Reflector. 49 



before ; at least there appeared to be no sensible diminution in 

 the brightness of the object, and the image was equally accurate 

 and distinct ; so that, if economy were a particular object aimed 

 at in the construction of these instruments, two good telescopes 

 might be obtained from one speculum. I have also some 

 reason to believe, that instruments of this kind might be 

 fitted up with glass specula. My opinion on tliis point, how- 

 ever, is not quite decisive, owing to the want of glass specula of 

 a proper figure and focal distance, on which to try the experi- 

 ments. With a glass speculum, however, about 11 inches focal 

 distance, and whose figure was obviously somewhat incorrect, I 

 have seen distant objects tolerably distinct and weU-defined, with 

 magnifying powers of from 12 to 20 times. 

 ,/ From the experiments I have made in reference to these in- 

 struments, it is demonstrable, that a tube is not necessary in the 

 construction of a reflecting telescope, whether it be used for 

 viewing celestial or terrestrial objects ; and, therefore, were a re- 

 flecting telescope of 50 or 60 feet in length to be constructed, it 

 might be fitted up at a comparatively small expence, after the 

 charge of casting, grinding, and polishing the speculum is de- 

 frayed. The largest instrument of this description which has 

 hitherto been constructed is the 40 feet reflector of Dr Herschel. 

 This compHcated and most unwieldy instrument has a tube of 

 rolled or sheet iron 39 feet 4 inches in length, and 4 feet 10 inches 

 in diameter, which weighs several thousands of pounds ; and it 

 has been computed that a wooden tube, for the same purpose, 

 would have exceeded this in weight by at least 3000 pounds. 

 Now, I conceive, that such enormous tubes in instruments of 

 such dimensions, are altogether unnecessary. Nothing more is 

 requisite than a short tube for holding the speculum. Connect- 

 ed with one side of this tube (or with both sides were it found 

 necessary), two strong bars of wood, projecting a few feet be- 

 yond the speculum end of the tube, and extending in front as 

 far as the focal length of the mirror, and connected by cross bars 

 of wood, iron, or brass, would be quite sufficient for a support 

 to the eye-piece, and for directing the motion of the instrument. 

 A telescope of 40 or 50 feet in length, constructed on this plan, 

 would not ^require one-fifth of the expence, nor oixe-fourth of the 

 apparatus .aAd^n^echamcalpa^^ it to any required 



APRIL JULY 1826, ■' ^' D 



