PORTION OF THE COSMOS. VISUAL POWER. 65 



frequent and highly serviceable : they are sometimes usefully 

 applied even to the measurement of double stars. Struve ( 125 ) 

 remarks, that with the Dorpat refractor, employing a magni- 

 fying power of 320, he had determined the smallest distances 

 apart of exceedingly faint double stars during a twilight so 

 bright that one could read conveniently at midnight. The 

 Pole star has a companion of the 9th magnitude only 18" 

 from itself ; in the Dorpat refractor Struve and "Wrangel 

 have seen this companion in the day-time, ( 126 ) as have also 

 (once) Encke and Argelander. 



The reason of the powerful effect of telescopes at a time 

 when, by multiplied reflection, the diffused light ( 127 ) of the 

 atmosphere is injurious, has given occasion to a variety of 

 doubts. As an optical problem it was regarded with the 

 most lively interest by Bessel. In his long correspondence 

 with me he often returned to the subject, and acknowledged 

 that he could find no solution which was entirely satisfactory 

 to him. I think I may reckon on the thanks of my readers 

 for the introduction in a note ( 128 ) of the views of Arago, as 

 contained in one of the many manuscripts which I was per- 

 mitted to use when at Paris. According to his ingenious 

 explanation, high magnifying powers facilitate the finding 

 and recognition of the fixed stars, because, without sensibly 

 enlarging their image, they conduct a greater quantity of in- 

 tense light to the eye, whilst they act according to a different 

 law on the aerial field from which the star detaches itself. 

 The telescope, by magnifying the distance between the illu- 

 minated particles of air, darkens the field of view, or dimi- 

 nishes the intensity of its illumination; and it is to be 

 remembered that we see only by the difference between the 

 light of the star and that of the aerial field, i. e. the mass of 



VOL. III. F 



