ASTRONOMICAL PHOTOGRAPHY — ^MEES 213 



recorded as much fainter than they appear to the eye, and conse- 

 quently astronomical knowledge has been built up chiefly in reference 

 to the brighter and bluer stars. 



With the availability of the new types of panchromatic and red- 

 sensitive plates, the exploration of the red stars of the universe is 

 yielding very valuable results. The use of red-sensitive materials 

 in astronomy has very definite advantages over the use of blue-sensi- 

 tive materials. The scattered light from the night sky is largely of 

 short wavelength because the scattering is selective, as is shown by the 

 color of blue sky, and much more effective exposures can be made on 

 stars before the scattered light from the sky buries the images in fog 

 if the photographs are taken by red light. The limit of effective 

 exposure by violet light, for instance, in the Mount Wilson telescopes 

 is approximately 90 minutes, after which little is gained because of 

 the fogging of the plate by scattered light. Using red-sensitive ma- 

 terials, the corresponding exposure can be more than five times as 

 great. Also there are many regions of the sky which are obscured, 

 and this obscuring material transmits red light much better than it 

 transmits the blue light. 



Plate 1 shows photographs of one of the most heavily obscured 

 globular clusters, which is situated close to the center of our galaxy. 

 Figure 1 was taken by W. Baade on a fast blue-sensitive plate with 

 50 minutes' exposure. Figure 2 shows the same cluster taken on a 

 red-sensitive plate with 75 minutes' exposure through a yellow filter. 

 These pictures show that the heavily obscured clusters are strongly 

 reddened and that the reddening not only affects the clusters but the 

 whole stellar field in which they are imbedded. 



The greater penetrating power of the red-sensitive plate enables 

 us to pass through the outer extensions of the hidden galactic nucleus 

 when photographing regions near the galactic center, which are ap- 

 parently little affected by obscuration. The new types of sensitizers 

 have made it possible to photograph not only the visible red but to 

 penetrate beyond it into the invisible infrared. J. J. Nassau, at the 

 Warner and Swazey Observatory, has found that many very red 

 stars, which are often variable, have absorption bands in the infrared. 

 Much interesting information is being obtained as to the nature of 

 these stars, especially as components of binaries and as to the part they 

 play in the nearer galaxies external to our own Milky Way system. 

 Moreover, the use of infrared spectra has given a good deal of informa- 

 tion concerning the planets. It is possible to set an upper limit, for 

 instance, to the concentration of water vapor in the atmosphere of 

 Mars and to show the concentration of carbon dioxide in the atmos- 

 phere of Venus, and by means of their absorption lines ammonia and 

 methane have been identified in the atmospheres of the outer planets. 



