526 BIBLIOGRAPHY OF HERSCHEL's WRITINGS. 



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



A. D. Vol. P. 



1782 72 96 [Foot-note.] By a set of experiments made in 1774 I found that I 

 could discover or perceive a bright object, such as white paper, 

 against the skylight when it subtended an angle of 35", but could 

 only distinguish it to be a circle, and no other figure, when it ap- 

 peared under an angle of 2' 24". 

 96 We must look for distinctness in the perfection of the object specu- 

 lum of a telescope, and if we can make the first image in the 

 focus of a speculum almost as perfect as the real object, what 

 should hinder our magnifying but loss of light ? 



96 The stars having light sufficient, I see no reason why we should 



limit the powers of our instruments by any theory. 



97 In this research it became necessary to look out for proper stars. I 



took some pains to find out what double stars had been recorded by 

 astronomers, but my situation permitted me not to consult exten- 

 sive libraries. Nor indeed was it very material ; for, as I intended 

 to view the heavens myself, Nature, that great volume appeared to 

 me to contain the best catalogue upon this occasion. 

 97-8 Enumeration of a few double stars which were known when this 

 research began. 



98 If I should mention any observations that are difficult to be verified 



I beg the indulgence of observers. I hope it will be found that I 

 have sufficiently guarded against optical illusions, and that I have 

 all along had truth and reality in view as the sole object of my 

 endeavors. 



99 In the observations of the distances of double stars, I have used two 



ways. Those which are extremely near to each other may be esti- 

 mated by the eye in measures of their own apparent diameters. 

 Such estimations are accurate, as is shown by experiments with 

 circles draw^n on paper near together. 



100 I have di\nded double stars into several classes. Class I contains the 

 most difficult. 



100 These being the most delicate objects it will not be amiss to go grad- 

 ually through a few preparatory steps of vision. 



100 Examples of such preparatory steps given. 



101 These double stars are a most excellent way of trying a telescope. 



10 L Class II of double stars contains all those that are proper for estima- 

 tions by the eye, or very delicate measures of the micrometer. See 

 fig. 6 of a Geviinorum. 



102 Estimations made with one telescope cannot be compared to those 



mada ■sith another. 

 102 Whatever may be the cause of the apparent diameters of the stars, 

 they are certainly not of equal magnitude with the same powers 

 in difterent telescopes, nor of proportional magnitude with dift'er- 

 ent powers of the same telescope. 



102 Class III contains all double stars more than 5" and less than 15" 



asunder. 



103 Class IV contains double stars 15 ' to 30" asunder. 

 Class V contains double stars 30" to 60" asunder. 

 Class VI contains double stars 60" and over asunder. 



103 These may serve another very important end. I will just mention it, 

 though it is foreign to my present purpose. Several stars of the 

 first magnitude have been observed or suspected to have a proper 

 motion of their own ; hence we may surmise that our sun, with 



