206 



STELLAR PHOTOGRAPHY. 



Another series of photographs is made of the polar regions. These are taken 

 at intervals of one hour in right ascension. Three regions are photographed on 

 each plate. The first extends from +65° to +75°, and has an exposure of 30', an 

 interval of 30 s , and an exposure of 120 s . The second extends from +75° to +85°, 

 and has an exposure of 30 s , an interval of 30 s , and an exposure of 240 s . The third 

 region extends from +75° to +85°, and is below the pole. The exposures are the 

 same as in the first region, but as the stars are moving in the opposite direction, 



the short line now comes on the opposite side of the long one. The regions 

 observed at lower culmination serve to correct the scale for portions of the sky 

 differing by twelve hours in right ascension. They also serve to determine the 

 law regulating the atmospheric absorption. This is, however, much better deter- 

 mined by the next series of photographs. Three regions are photographed on 

 each plate, all extending from +55° to +65°. The first is on the meridian above 

 the pole, and is made by an exposure of 10 s , an interval of 10 s , and an exposure 

 of 40 s . The second region has the same exposure, but is taken below the pole 

 and at an hour angle of thirty minutes west, that is, it contains stars that have not 

 yet culminated. The third region is also below the pole thirty minutes east. Two 

 equal exposures of 30* each are given, separated by an interval of 10*. These 

 plates are taken at intervals of twenty minutes in right ascension, and in general 

 one is exposed each night. Two regions are photographed at lower culmination for 

 one at upper culmination, because the stars form so much fainter images when low 

 that comparatively few are obtained at each exposure. 



The number of stars shown in these plates, especially in those relating to the 

 polar regions, is very large. Even if it should prove impracticable to identify and 

 measure them all, they will have a value as a permanent record of the condition 

 of the sky at the present time. An illustration of this occurred in Plate 117, which 

 was taken on November 9, 1885. It included the region in which the new star 

 in Orion was discovered on December 13. No evidence of the new star is visible, 



•- 



h the adjacent stars DM. +19° 1106 and DM. +20° 1156 are so well 



as to be easily seen in a paper positive. The magnitudes of these stars in the 

 Durchmusterung are 6.8 and 7.2 respectively. After allowing for the difference in 



between these stars and the 



evident that the latter must 



been much fainter on November 9 than at the date of discovery. It is believed that 

 no other positive evidence of this fact has been shown to exist. 



