420 BARNARD— ASTRONOMICAL PHOTOGRAPHY [April 20, 



graphic plate, not only with an accuracy far beyond that of the 

 most skillful artist, but with an eye almost infinitely more sensitive, 

 sees the faintest details of a comet or a nebula and records them 

 with a faithfulness unheard of before. 



Today the sensitive plate is not only taking the place of the 

 astronomical draughtsman, but it is also running the most skillful 

 measurer a close race. The facility and ease with which great 

 numbers of star places can be measured on the photographic plate, 

 commend it to the most exacting astronomer. 



Photography has materially altered our ideas of the nebular 

 theory. From the views of the nebulae with telescopes not suffi- 

 ciently powerful to properly deal with them, and hence with views 

 that were more or less erroneous, a theory was elaborated that 

 appealed to the popular mind with a wonderful fascination. There 

 is much that must be changed in this theory to meet the rigid re- 

 quirements of modern science and to satisfy the demands of what 

 has been revealed in the forms of the nebulae by the photographic 

 plate. 



It is in dealing with the nebulae that astronomical photography 

 has attained one of its most remarkable triumphs. These bodies in 

 reality shine with a light that has comparatively little effect on the 

 human eye but to which the photographic plate is singularly sensi- 

 tive. To our eyes the nebulae are seen "through a glass darkly," 

 as Jt were, while to the eye of the sensitive plate they are more or 

 less brilliant objects. 



Our old ideas of the dimensions of these vast bodies have also 

 been greatly changed. In the days of purely visual astronomy, the 

 great nebula of Orion, covering as it does some half a degree of the 

 sky, was looked upon as inconceivably great in actual extent in space 

 • — yet photography has not only increased its extent very greatly, 

 but it has revealed other nebulae, unknown to us before, that are 

 hundreds of times vaster than this great nebula of Orion. The 

 Pleiades are in the midst of a mighty system of nebulosity that covers 

 at least one hundred square degrees of the sky, and whose actual 

 extent in space almost defies calculation. 



Four or five degrees north of the star Antares, in the Scorpion, 

 is a faint star just fairly visible to the naked eye. This is known 



