BIBLIOGPAPHY OF HERSCHEL'S WRITINGS. 537 



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



A. B. Vol. p. 



1785 75 41 Method of identifying and describing the positions of the stars. 



43 I would recommend a precaution to those who wish to examine the 



closest of my double stars. It relates to the adjustment of the focus. 

 Supposing the telescope and the observer long enough out in the 

 open air to have acquired a certain temperature, and the night suf- 

 ficiently clear, let the focus be readjusted with the utmost delicacy 

 upon a star known to be single, of nearly the same altitude, magni- 

 tude, and color as the star to be examined. 



44 Let the phenomena of the adjusting star bo well attended to, as to 



whether it be perfectly rounded and well defined, or affected with 

 little appendages, etc. Such deceptions may be detected by turn- 

 ing or unscrewing the object glass a little in its cell, when those 

 appendages will be observed to revolve the same way. 



44 Being thus acquainted with the imperfections as well as the perfec- 



tions of the instrument, and going immediately from the adjusting 

 star, we may hope to be successful. 



45 All the observations here given were made with a power of 460 unless 



they are marked otherwise. 



45 The measures were all taken with a parallel silk-worm's-thread mi- 



crometer and a power of 227 only, from the center of one star to the 

 center of the other. 



46 The threads subtend an ansle of 1" 13'" only. 



46 The positions have all been measured with a power of 460 on a mi- 



crometer made for me according to the model given in the Phil. 



Trans., vol. Ixxi, page 500, fig. 4. 



W. Herschel. 

 [Dated] Datchet, near Windsor, Nov. 1, 1784. 



47 Catalogue of Double Stars, First Glass. [Nos. 25 to 97.] 



48 [Foot-note. ] Could we increase our power and distinctness at pleasure 



we might undoubtedly separate any two stars that are not abso- 

 lutely in a direct line. * * * This will appear when we con- 

 sider that perhaps f § of the diameter of a star are spurious. It 

 would have been curious if a considerable difference in the colors 

 could have led us to discover which of the two stars is before the 

 other ! But by far the greatest part of their diameters being spu- 

 rious, it is probable that a different-colored light of two stars would 

 join together where the rays of one extend into those of the other; 

 and so, producing a third color by the mixture, still leave the ques- 

 tion undecided. 



51 [Foot-note.] The interval between very unequal stars estimated in 

 diameters generally gains more by an increase of magnifying power 

 than the apparent distance of those which are nearer of a size. 

 However, this only seems to take place when there is a difficulty of 

 seeing the object well with a low power. 



65 Second Class of Double Stars. [Nos. 39 to 102.] 



[Foot-note, j When the small star is so faint as not to bear the least 

 illumination of the wires, its position may still be measured by the 

 assistance of some wall or other object ; for an eye which has been 

 some time in the dark can see a wall in a starlight night sufficiently 

 well to note the projection of the stars upon it in the manner which 

 has been described with the lamp micrometer. Then introducing 

 some light, and adapting the fixed wire to the observed direction 



