92 THE KINGDOM OF MAN 



Astronomy. A biologist may well refuse to offer any 

 remarks on his own authority in regard to this earliest 

 and grandest of all the sciences. I will therefore at once 

 say that my friend the Savilian Professor of Astronomy in 

 Oxford has turned my thoughts in the right direction in 

 regard to this subject. There is no doubt that there has 

 been an immense ' revival ' in astronomy since 1881 ; it 

 has developed in every direction. The invention of the 

 ' dry plate/ which has made, it possible to apply photo- 

 graphy freely in all astronomical work, is the chief cause 

 of its great expansion. Photography was applied to 

 astronomical work before 1881, but only with difficulty 

 and haltingly. It was the dry-plate (see Fig. 12) which 

 made long exposures possible, and thus enabled astrono- 

 mers to obtain regular records of faintly luminous objects 

 such as nebulae and star-spectra. Roughly speaking, the 

 number of stars visible to the naked eye may be stated as 

 eight thousand : this is raised by the use of our best 

 telescopes to some hundred million. But the number 

 which can be photographed is indefinite and depends on 

 length of exposure : some thousands of millions can 

 certainly be so recorded. 



The serious practical proposal to * chart the sky ' by 

 means of photography certainly dates from this side of 

 1881. The Paris Conference of 1887, which made an 

 international scheme for sharing the sky among eighteen 

 observatories (still busy with the work, and producing 

 excellent results), originated with photographs of the 

 comet of 1882, taken at the Cape Observatory. 



Professor Pickering, of Harvard, did not join this 

 co-operative scheme, but has gradually devised methods 

 of charting the sky very rapidly, so that he has at 

 Harvard records of the whole sky many times over, and 

 when new objects are discovered he can trace their 



