MR. DOLLOND ON A CONCAVE ACHROMATIC LENS. 201 



" 5th. The definition of the stars seems quite as good ; and the false light does not 

 appear to be increased, or the regularity of its distribution affected. The discs of the 

 stars seem in fact to be, if anything, rather smaller and cleaner with the concave. 

 Perhaps their brightness might be perceived to be a trifle less; but even this is doubt- 

 ful. See below. 



^^ 6th. The shallower eye-glasses are much more easily cleaned ; of great importance 

 in high powers. 



" 7th. The prism can be conveniently applied to all powers as high as 600 ; before, 

 only to 285. This prism is of essential utility in other respects besides facilitating 

 zenith observations ; and it is no small improvement that its use is thus extended. 



" From the performance of this additional lens, it is evidently a perfect production. 

 Against all the advantages detailed above, the trifling addition to the length of the 

 telescope is not to be mentionisd ; indeed it is to me surprising that so great an effect 

 should be produced with so minute an increase of focus. 



" As a severe trial of the difference in illuminating power, I have examined Saturn's 

 satellites, and it Geminorum. I could discover no decided difference in the apparent 

 brightness of the satellites, allowance being made for the difference of power employed. 

 It happens awkwardly, that among moderate powers, fit for planets, none coincide 

 sufl[iciently with and without the concave lens. The nearest I can get are a negative 

 195 with the new lens, and a double convex 208 without it : with these, little difference 

 in brightness ; but the planet might be a trifle sharper with the latter. Have you ever 

 seen the minute companion of k Geminorum ? It is the finest test of a five-feet 

 achromatic I have yet seen : distance about 6". I saw it steadily with negative 140 

 without the concave, and quite as well with negative 116 with it ; but these powers 

 are not near enough to each other. 



" For tolerably bright stars, I have on the micrometer 475 with the concave lens, and 

 without it 480; also 600 with, and 625 without. These afford an excellent comparison. 

 Vision appears to me equally good with both ; and the fineness of the micrometer 

 threads leads me always to prefer the new arrangement, as I can then use the same 

 eye-piece generally for the distances, as I use for the positions. 



" In clear weather, I always use 600 for stars of the fifth magnitude and upwards, 

 and sometimes even of the sixth ; and last night I got a very good set of positions of 

 Castor with a power of 1010, with which the discs were occasionally perfectly well 

 formed, though of course not so sharply defined. I also obtained last night very satis- 

 factory measures of ^ Cancri, certainly one of the most difficult stars for a telescope 

 of five feet. That you may judge for yourself of the way in which it was seen, I will 

 detail here my measures, exactly transcribed from my observation paper. 



Power. Position. 



600 336° 56' Mean = 335° 28' 



Stars placed between the 335 50 z = — 271 26 



parallel wires thus : 336 22 • 



335 8 64 2 nf. 



335 7 



334 1 = 25 58 from N. 

 334 49 



MDCCCXXXIV. 2 D 



