192 STELLAR PHOTOGRAPHY. 



formed by a lens of long focus at once shows itself An increase in the angular 

 aperture is, however, a great advantage in photographing a faint surface. 



When the different portions of a plate are subjected to a gradually increasing 

 lio-ht, up to a certain point no perceptible effect will be produced. The darkening 

 then becomes more and more intense, until a certain maximum effect is produced, 

 and then with a very intense light, like that of the Sun, a reversing action takes 

 place, by which the density becomes less and less. Representing by a curve tlie 

 relation between the darkening and the total actinic energy received upon any 

 portion of the plate, we find that these curves may differ in three essential partic- 

 ulars. First, the amount of light required to produce the first impression upon the 

 plate. This may be regarded as a measure of the sensitiveness of the plate, or its 

 value for photographing very faint objects. Secondly, one curve may be steeper 

 than another; that is, the increase in darkening may be more marked with one 

 plate than another, with a given increase of light. The greater this increase, the 

 better is the plate adapted to show differences in the light of stars of nearly equal 

 magnitude, or to show details in nebulte or spectra. On the other hand, the range 

 of such a plate is small, and it will be less adapted for making charts or other picto- 

 rial representations. Moderately bright stars will completely decompose the silver 

 particles, and cannot be distinguished from much brighter objects. The brighter 

 portions of a nebula or spectrum will also be burned out, and will fail to show great 

 variations in light as w^ell as plates less sensitive to small changes. Thirdly, the 

 maximum darkness of different plates also varies ; but this is a matter of less im- 

 portance for our present purpose. A fuller consideration of this subject, with 

 measures of the constants of various plates, will be given by Mr. W. H. Pickering 

 in the Proceedings of the American Academy. 



In view of the continual improvement in photographic processes, and the 

 increase in sensitiveness that has been attained in the more recent forms of plates, 

 it becomes an interesting question to consider what is likely to limit the results 

 attained. We are already approaching this limit on moonlight nights. The fogging 

 of the plates is so great when the moon is nearly full, that long exposures cannot be 

 used with a telescope of so large an angular aperture as the telescope here employed. 

 A further increase in the sensitiveness of the plates will render it impossible to work 

 to the best advantage in the vicinity of a large city, on account of the illumination 

 of the atmosphere by artificial light. It will then be necessary to take the photo- 

 graphs in more remote regions, or preferably at great elevations, where the reflect- 

 ing atmosphere is diminished in amount. For these reasons a great increase in 



