No\-KMBKR 1, 1900.] 



KNOWLEDGE. 



219 



The cvliudricitv of the axes and rollers of the siderostat 

 was verified with an accuracy of ^TTihnj *'f "■° inch. 



The diameter of the object glass, which is a photo- 

 graphic one, measures, as above stilted, 49.2 inches, 

 and its weight is 794 lbs. But the clear aperture is 

 47.2 inches, and the focal length some 187 feet. Hence 

 the photogi'aphic images of the sun or moon in the 

 primaiy focus measure from 21 to 22 inches across. 



The tube of the telescope is 180 feet long, and 59 

 inches broad. It is of steel, rather less than jV, inch 

 thick, and weighs 21 tons. The total weight of the 

 instrument, including the siderostat, thus falls but little 

 short of 70 tons. The tube rests on five cast iron 



movements of the siderostat, 250 feet off, with whom, 

 however, he can communicate telephonically. 



Compared to the Yerkes telescope, the light-grasping 

 power of the Paris refractor is as about 2r^ is to 2, in 

 favour of Pai'is, due allowance being made for the loss 

 of light (8 per cent.) by reflection ou the silvered mirror. 

 The stellar penetration of the siderostat ought, therefore, 

 to i-each the ISth magnitude. 



It was through M. Flammarion's kiiuhiess that the 

 writer was enabled to utilize the Paris siderostat. 



The planets Jupiter and Saturn were unfortunately 

 out of reach beyond 20° of south declination, inasmuch 

 as it was not thous;;ht safe In' the maker to allow tlio 



FlQ. 6. — Gentral A'iew, ihowicg the cyL-cnc', of tlie Great Tclespopr, as nioiinliil in tlie Palais de rO]iliiiiU' at tiif Paris Exliiliition. 



supports, besides the two other supports, one at each 

 end. 



A short tube, of the same breadth as that of the 

 telescope, but resting on four wheels, forms the eye end. 

 The wheels can glide along a railway, so as to facilitate 

 the focussing of the plate or eyepiece, which would 

 otherwise be extremely inconvenient, seeing that the 

 weight of this eye end is also counted in tons. 



Fig. 6 gives a general view of the telescope with 

 the eye end, as mounted in the Palais de I'Optique, 

 Champ de Mars, at the Exhibition. 



All heavenly bodies have to be found by their right 

 ascension and declination. There is no possibility of 

 directing the mirror's motion from the eye end. Hence 

 the helpless observer at the eyepiece is to some extent 

 " at the mercy " of the astronomer in charge of the 



mirror to make a smaller angle than 12'^ with the 

 vertical. 



Venus, however, was well situated during the summer, 

 and the writer was enabled to secure a considerable 

 number of drawings of her at daytime. The great 

 telescope showed the planet utterly destitute of detail 

 (Fig. 7). I Its appearance was that of a pale yellow 

 crescent or half-moon, with a brighter limb, projected on 

 the dark azure of the sky. Hence the inanity of all 

 rotation periods based on tiic observation of subjective 

 spots, and fixed sometimes (as in the case of M. Brenner) 

 with the accuracy of one ten-thousandth of a second I 



X It will be noticed tliat owing to tlie mirror'ii reflection, the images 

 are inverted east and west. The focus, nioi-eover, for Venus in the 

 great refractor was some ten or fifteen inches fartlier out than that of 

 stars or nehuhe. 



