BIBLIOGRAPHY OF HERSCIIEL's WRITINGS. 525 



Hersehel, W.: S\'^"orsiS OV the Wiutin-gs of— Continued. 



A. D. Vol. p. 



17«ii 72 83 Galileo was the first who suggested this method ; but ho does not 

 observe that the stars shouhl be so near to each other as thereby to 

 preclude the inHuence of every cause of error. 

 88 This method has been also mentioned by other authors. Dr. Lon'g 

 observed y ArictLf, a Geminorum, Ononis, and y Virginia, and " was 

 persuaded that they woukl be found always to remain the same." 

 88 Every one of these stars are [sjc] totally improper for the purpose ; for 

 the stars in y Arictis are near 10" distant, and, moreover, equal in 

 magnitude. In a Geminorum the stars are near enough, but do not 

 sufficiently dilier iu magnitude to shew any parallax. The stars 

 in the Nebula of Orion, on account of their extreme smallness of 

 distance, are still more improper than any; and those of y Viryinis 

 are equal in magnitude. 



88 The magnifying powers used by Galileo and LoxG were too small, 



not above 60 or 70. 



89 From a great number of observations which I have already made on 



several double stars, especially e Bootis, it appears that we can athrm 

 the annual parallax to be exceedingly small indeed ; and that there 

 is a great probability of succeeding still farther in this laborioug 

 but delightful research, so as to be able at last to say, not only how 

 much the annual i>arallax is not but how much it really is. 



90 Discussion of the effect of refractiou on such measures. 



91 Too much has hitherto been taken for granted in optics. TMiy the 



method (of experiment) should not be more pursued in the art of 

 seeing does not appear. 



92 We are told that we gain nothing by magnifying too much. I grant 



it ; but shall never believe I magnify too nnich till by experienco 

 I find that I can see better with a lower power. 



92 Telescopes will in general discover more small stars the more light 



they collect, yet with a power of yS7 I cannot see the small star 

 following o Aquilce, when by the same telescope it appears very 

 plainly with the power of 4G0. Now in the latter case the light 

 is less than the fourth part of the- former. 



93 Other similar cases noted and the names of the small stars given. 



93 Great power may be favorable in cases where two stars are close 



together. Figs. 2-5 show a Lyrw with powers 460, 2,010, 3,168, 

 and 6,450 with my Newtonian reflector. 



94 A new micrometer has been invented, which will be described in a 



subsequent paper. 



94 The powers that may be used upon various double stars are different 



according to their relative magnitudes; s Bootis, for example, 

 will not bear the same power as a Geminorum. 



95 I have always found a single eye-glass had much the superiority 



over a double eye-glass, both in light and distinctness. I would 

 except those cases where a large field is necessary. 

 95 If we would distinctly perceive and measure extremely small quan- 

 tities, such as a tenth of a second, it appears that when we use a 

 power of 460 this tenth of a second will be no more in appearance 

 than 46", and even with a power of 1,500 will be but 2' 30", which 

 is a quantity not much more than sufficient to judge well of ob- 

 jects and to distinguish them from each other, such as a circle 

 from a square, triangle, or jiolygon. 



2h 



