BIBLIOGEAPHY OF HERSCIIEL's WRITINGS. 525 



Herschel, W.: Synopsis of the Writings of— Continued. 



A.B. Vol. P. 



I~ii2 7^ SdS Galileo was the first wlio suggested this method; but he does not 

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

 preclude the intiuence of every cause of error. 

 88 This method has been also mentioned by other authors. Dr. Long 

 observed y Arieiis, a Geminorum, Oriotiw, and y Virgims, and " was 

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

 83 Every one of these stars are [sj'c] totally improper for the purpose ; for 

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

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

 sntiiciently ditier iu magnitude to shew any jiarallax. The stars 

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

 distance, are still more improiier than any ; and those of y Virginis 

 are equal in magnitude. 



88 The magnifying iiowers used by Galileo and Long 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 affirm 

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

 is a great probability of succeeding still farther in this laborious 

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

 much the annual parallax is not but how much it really u. 



90 Discussion of the effect of refraction on such measures. 



91 Too much has hitherto been taken for granted iu oi)tics. Why the 



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

 seeing does not appear. 



92 We are told that we gain nothing by magnifying too much. 1 grant 



it ; but shall never believe I magnify too much till by experience 

 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 S«i7 I cannot see the small star 

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

 plainly with the power of 460. 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 Lgrcc 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 



subsaquent paper. 



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



according to their relative magnitudes; £ 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 4()0 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 i)olygon. 



2h 



