STELLAR PIIOTOGIlArilY. 



211 



was exposed for 5 minutes with the clock on, then the telescope was moved 1° in 

 declination, and exposed for 1 minute with the clock on again. The clock was 

 then stopped, and after an interval of 30 seconds a trail was formed for 30 seconds. 



given witli 



Plate 248 was taken on January 6, 1886. An exposure of 10 minutes was 

 the clock on, and this was followed by a trail of 1 minute. Plate 327 was taken 

 on January 23, 1886, without clockwork. Eight exposures were given, having the 

 lengths 10 s , 5% 2 s , 1% I s , I s , 0\5, 0'.5, 0*.5, and 40 s respectively. The objective 

 was covered for a few seconds between each exposure. Unfortunately, in putting 

 on the cap, a slight pressure on the telescope brought the images slightly out of 

 line. Plate 361 was taken in a similar manner on February 9, 1886, but a 

 greater number of exposures of various lengths were made. From Plate 3 



i 



appeared that some stars, the trails of which were visible when the exposure was 

 sufficiently long, produced no effect with an exposure of a single second. The 

 experiment was therefore tried with Plate 361, during a portion of the time, of 

 making short breaks between exposures, lasting for several seconds. Measures 

 could thus be made of the positions of much fainter stars. 



A scale for measuring the relative intensities of these trails was constructed 

 from No. 15. This photograph was taken, August 15, 1885, by pointing the tele- 

 scope to the north pole, and giving a series of exposures of ten minutes each with 

 different apertures. The clockwork was detached, so that the telescope was at rest, 

 and the aperture was varied by inserting a series of circular diaphragms, having 

 diameters of 2.07, 3.12, 4.92, 7.77, 12.33, and 19.55 cm. The last gave nearly the 

 full aperture of the telescope, the others reduced it by quantities which respect- 

 ively correspond to 4.86, 3.99, 3.00, 2.01, and 1.00 magnitudes. Each star accord- 

 ingly left a trail consisting of six short lines connected together at the ends, and 

 each representing a difference in light from that next it of one magnitude. The 

 smallest aperture was however too large by about 0.1 cm., so that the interval 

 between it and the preceding, expressed in magnitude, was only 0.87. The por- 

 tion of the plate containing the trails of 8 Ursa Minor is and 24 Una Mwris was 

 cut out and attached to a piece of cardboard, so that it could be laid upon the image 

 of the star to be measured. The brightest portion of the trail of S Ursw Minoris wa« 

 assumed to represent the magnitude 4, and the faintest portion the magnitude 9. 

 All of these results have been diminished by one magnitude to make the scale agree 

 more nearly with that in common use. Each trail to be measured was compared 

 directly with this scale, the fraction of a magnitude being estimated to tenths. 

 After measuring in this way all the trails visible on each of the plates, the results 



