ASTRONOMICAL PHOTOGRAPHY — MEES 217 



Ni and Na- It has been shown that no such element as nebulium exists 

 and that these lines are the result of a very unusual state of emission by 

 oxygen, known as a forbidden state. In a wavelength region isolated 

 in the second picture, recorded on a 103a-D plate through a GG-14 

 filter, no emission from the gases in the nebulosity is recorded. Only 

 stars are shown. In the third picture, taken on a 103a-E plate through 

 an RG-2 filter, only the emission of the H-alpha line is recorded in 

 the nebulosity together with the red light of the stars. This series 

 shows the powerful tool afforded by plates of different color sensitivi- 

 ties combined with suitable filters in the study of the structure of 

 extragalactic objects. 



During the last few years, a number of new telescopes have come 

 into use, and their effective use presents some new problems in regard 

 to the photographic materials, though their application is already 

 giving results of great interest and value. By far the greatest of 

 these telescopes is, of course, the 200-inch Hale telescope at Mount 

 Palomar, of which you have already heard so much. Besides this in- 

 strument, however. Mount Palomar has the largest Schmidt telescope, 

 the 48-inch, with which an excellent survey of the sky can be made, 

 covering a much greater field than w^as available for previous tele- 

 scopes. The great 48-inch Schmidt is being used in a survey of the 

 whole sky accessible to it, using plates 14 inches square sensitive to 

 blue light and to red light. Sufficient plates for a year's use are made 

 to ensure uniformity of material, and they are stored at a low tem- 

 perature so that they can be expected to remain unchanged until they 

 are used. Plate 6 is one of the latest pictures of the great Andromeda 

 Nebula made with an exposure of 35 minutes on the Schmidt telescope. 

 The excellent definition of this instrument resolves many stars in the 

 spiral; the two small nebulae are the well-known satellites of the 

 Great Nebula. 



Employing 103a-E plates with a red filter on the Hale telescope, 

 Baade was able to resolve one of the companions to the Andromeda 

 Nebula into its individual stai-s. With other combinations of plates 

 and filters, the stars of different colors were separated and their dis- 

 tribution centered. This led to the startling discovery that there 

 are two types of stars in the nebulae. These are called by Baade 

 Population I and Population 11^ and they exist in different propor- 

 tions in different galaxies. Type I population is composed of stars 

 that are intrinsically very bright and generally blue in color. A 

 blue-sensitive plate primarily records these stars. Population II 

 stars are intrinsically fainter and redder. These are recorded on 

 red-sensitive plates. 



The most sensational discovery of the astronomers in this century 

 has undoubtedly been the announcement by E. P. Hubble of Mount 



