PORTION OF THE COSMOS. PHOTOMETRY. 85 



of being regarded as a great and serious object of scientific 

 investigation. This improvement can alone render it possi- 

 ble to leave to future generations numerical determinations 

 respecting the light of the heavenly bodies. Many geolo- 

 gical phsenomena which connect themselves with the thermic 

 state of our atmosphere, and relate to the former distribution 

 of plants and animals on the surface of our globe, may be 

 elucidated thereby. More than half a century ago such 

 considerations had not escaped the great investigator William 

 Herschel, who before the close connection between electricity 

 and magnetism had been discovered, compared the ever 

 luminous cloud-envelopes of the solar orb, to the Polar Light 

 of the terrestrial globe. ( l64 ) 



Arago recognised in the complementary condition of co- 

 loured rings seen by transmission and reflection, the most pro- 

 mising means of a direct measure of the intensity of light. I 

 give in a note( 165 ) in my friend's own words a statement of 

 his photometric method, to which he has also added the 

 optical principle on which his cyanometer rests. 



The so-called relative magnitudes of the fixed stars now 

 given in our Catalogues and Star Maps are partly belonging 

 to the same epoch, and partly include alterations of light 

 belonging to different epochs. We cannot, however, as was 

 long assumed, take as a safe criterion of such changes, the 

 succession of the letters of the alphabet appended to the 

 stars in the Uranometria Bayeri, which has been in such 

 extensive use since the beginning of the 17th century. 

 Argelander has shewn that we cannot infer relative bright- 

 ness from alphabetical order, and that Bayer allowed himself 

 to be guided in the choice of letters by the shape and 

 direction of the constellations. ( 166 ) 



