310 Mr South on the Relative Performances 



achromatic telescope, constructed by M. Lerebours, now in the 

 Royal Observatory of Paris, is rather better than 9.2 inches 

 English, uncovered by the cell, but of which 8.4 inches only 

 are in actual use. Its focal length is eleven feet. 



The magnifying powers with which I used it on the night of 

 the 15th of March last, are 136, 153, 224, 240, 420, and 560. 

 With all, except 560, (which, by some forgetfulness, was not 

 applied,) Venus was extremely well defined during a dark 

 night ; of course, Jupiter and Saturn were well shown. The 

 two stars of Castor, of y Leonis, of Z, Ononis, were exhibited 

 with 240, 420, and 560, as round as possible, u Leonis * pre- 

 sented by its side a light-blue star with 420, which could not 

 be overlooked by the most careless observer, and with 560 

 both stars were admirably defined. 



I need not inform you, that a telescope, having an object- 

 glass of the diameter above mentioned, which, with these pow- 

 ers, will neatly define the limb of the planet Venus, and will 

 give to the discs of the double stars here named images abso- 

 lutely round, deserves to be well spoken of. Indeed, I have 

 no hesitation in saying, that this telescope is the best achro- 

 matic I 'ever pointed to the Heavens ; nor will I withhold my 

 rrgret, or even the mortification I feel in asserting, that Eng- 

 land, when I visited it in May last, could not produce an 

 achromatic any thing like it. The stand upon which it is 

 mounted is not provided with any means of giving to the tele- 

 scope equatorial motion. 



Whilst, however, I say thus much, I am far from enter- 

 taining the sentiments of Mr Fraunhofer, as to the decided 

 superiority of refractors over reflectors, nor can I accompany 

 Mr Struve in his idea, that the Dorpat telescope " may per- 

 haps rank with the most celebrated of all reflecting telescopes, 

 namely, Herscl^l's." It is true, I have not had the enviable 

 qualification of having seen the former ; still I think the Paris 

 telescope furnishes me with data upon which to form some- 

 thing like a rational conjecture. 



Its object-glass actually in use is in proportion to Mr 

 Struve , s, (if all of it be effective,) as seventy to ninety-two near- 

 ly, a difference not very hard to be allowed for. 



" According to Professor Amici, the two stars are distant 0"..5 — En. 



