530 BIBLIOGRAPHY OF HERSCHEL'S WRITINGS. 



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

 A. D. Vol. P. 



1782 72 176 The inconvenience of the first method is that we find out how much 



the telescope should magnify rather than how much it really does 

 magnify. 

 177 To prevent any mistakes I wish to mention again that I have all along 

 proceeded experimentally in the use of my powers, and that I do 

 not mean to say I have used 6450 (or 5786) upon the planets, or even 

 upon double stars. The use of high powers is a new and untrodden 

 path, and in this attempt variety of new phsenomena may be ex- 

 pected, therefore I wish not to be in a haste to make general con- 

 clusions. I shall not fail to pursue this subject, and hope soon to 

 be able to attack the celestial bodies with a still stronger armament 

 which is now preparing. 



1783 73 \ A letter from William. Herschel, esq., F. R. S., to Sir Joseph Banks, 



Bart., P. R. S. 

 1 The name Georgium Sidus proposed for the new star discovered in 

 March, 1781. lUraiius.'] 



1783 73 4 On the Diameter and Magnitude of the Georgium Sidus; witha description 

 of the dark and lucid disk and periphery micrometers. By William 

 Herschel, Esqr., F. R. S. Read November 7, 1782. 

 5 The measures of the diameter of the Georgium Sidus formerly commu- 

 nicated give 4" 36|'". But not being satisfied, when I thought it 

 possible to obtain much more accurate measures, I employed the 

 lamp micrometer. 



5 A lucid disk and not two lucid points was really required, and the 



following apparatus was contrived. [Description follows.] The 



6 planet was kept by a good screw opposite and covering illuminated 

 oiled paper disks, and the sizes of these measured. 



7 Observations on the Light, Diameter, and Magnitude of the Georgium 



Sidus. [From Oct. 22, 1781, to Nov. 4, 1782.] 



7 Oct. 22, 1781, "had a fine, bright, steady light, of the color of Jw^ifer, 



or approaching to the light of the moon." 



8 Oct. 2, 1782. "The planet unexpectedly appeared bluish," while the 



oiled disk was reddish. 



9 Oct. 10, 1782, determined the magnifying power of the telescope 



again in a new way [described]. 



11 There is one cause of inaccuracy or deception in very small [close] 

 measures, long suspected but never yet sufiiciently investigated. 

 A dispersion of the rays of light in the atmosphere may be admit- 

 ted ; a concentration may also take place. 



11 Oct. 12, 1782. The planet visible to the naked eye. 



11 Oct. 13, 1782. "I perceived no flattening of the polar regions." 



12 Nov. 4, 1782. "I was now fully conviuced that light, be it in the 



form of a lucid circle or illumiuated jieriphery, would always occa- 

 sion the measures to be less than they should be, on account of its 

 vivid impression upon the eye" [and a dark circle in a bright 

 square was used for comparison]. 



12 A method to discover the quantity of the deception arising from the 



illumination pointed out. 



13 The diameter of the Georgium Sidus cannot well be much less, nor 



perhaps much larger, than about 4". 



