746 



HANDBOOK OF PHOTOGRAPHY 



been uniformly blackened. The illustration herewith reprinted by the courtesy of the 

 Eastman Kodak Co. shows these tracings for their plates as follows: 



Table I. — Resolving Power to White Light 



Material 



R. P. 



Material 



R. P. 



Eastman 50 



Eastman 40 



Eastman 33 



Eastman Process 



38 

 40 

 60 

 80 



Type I... 

 Type II.. 

 Type III. 

 Type IV. 

 Type 144 

 Type V. . 



45 

 50 

 70 

 85 

 80 

 160 



The column marked R. P. opposite the types of emulsions gives the corresponding 

 resolving power which may be defined as the number of lines (black and white of equal 

 width) per millimeter on the plate that may be fully resolved into separate entities. 

 The list is arranged in order of decreasing sensitivity in each of the two listings. 



Color Curves. — The selection of plates for photography with refracting telescopes 

 requires that a plate be employed with spectral sensitivity specially suited to the 

 region for which the lens of the refractor has its best field if star images of good defini- 

 tion are to result. This presupposes a knowledge of the color curve of the lens. The 

 color curve of the lens may be determined by finding the exact focus of a star for speci- 

 fied wavelengths as will be explained in a later section. 



Fig. 6. — Color-sensitivity curve for emulsion B. {Eastman Kodak Co.) 



For most visual refractors the flattest part of the color curve is in the neighborhood 

 of 5600 A. For photography with such a telescope, plates particularly sensitive to 

 this region should therefore be selected, such as the panchromatic plate of Class B or 

 Class C sensitizing. To prevent blurring of the stellar image by out-of-focus rays of 

 shorter or longer wave lengths, a yellow filter should be used in front of the emulsion, 

 such as the Wratten filter No. 12 which has been adopted by most of the major observ- 

 atories. The color curve of the panchromatic emulsion B represented above shows 

 fairly uniform sensitivity from wavelengths 5200 to 6400 A. 



The reflecting telescope has, of course, a great advantage in that there is very 

 little spectral selectivity in the reflection of light from a silvered or aluminized surface, 

 at least throughovit the spectral region transmitted by the atmosphere of the earth. 

 In astronomical work involving investigations of colors of stars, a variety of plate 

 emulsions maj^ be utilized by the same instrument in conjunction with appropriate 

 filters. This, of course, cannot be so effectively done with the refracting type of 

 instrument on account of the strong color characteristics of the lens. 



The existence of dark stars whose radiations are too far in the infrared to make an 

 impression on the ordinary plate have been recently found by Hetzler of the Yerkes 

 Observatory through the use of supersensitive panchromatic emulsions. Often the 

 sensitivity of these emulsions may be materially increased by hypersensitizing with 



