326 DISCOVERY CH. 



of celestial light of so refined a nature as to be impercep- 

 tible to unaided vision. Looking into the background of 

 infinity, the chemical retina of the photographic plate 

 may record nothing at first sight ; but let it remain with 

 steadfast gaze for an hour or so, and the accumulated 

 impressions produced by faint rays acting continuously 

 upon it will often show that there is indeed light in the 

 darkness of space. Stars and sheets of luminous material 

 have thus been depicted which no human eye has yet 

 been permitted to see directly. 



It may be assumed that new worlds will continue to 

 be brought into light as new methods and new instru- 

 ments are used in the study of the heavens. There are 

 reasons for believing, however, that the thousand million 

 stars which can be seen with our largest telescopes, or 

 registered upon photographic films, represent practically 

 all that are shining in our universe. 



Photographs of the streams and sheets of luminous 

 haze that exist in many parts of the heavens lead to the 

 conclusion that the starry universe visible from the 

 earth is limited in extent. Beyond a certain limit, long 

 exposures of sensitive plates to celestial light do not 

 reveal additional stars or nebulous realms ; and the 

 evidence suggests that, so far as number and extent are 

 concerned, the visible stellar universe has been completely 

 surveyed. 



But though the stars can be numbered, no sounding- 

 line has yet been imagined which will fathom the depths 

 of infinite space. The human intellect may, in the course 

 of ages, comprehend the whole of our universe, but 

 infinity is beyond its grasp ; and in boundless space 

 there may be many universes like that around us. 

 Measured with our finite minds, the distances of the 

 stars are too great for us to have an adequate conception 



