igoy] PERTAINING TO WORK WITH A PORTRAIT LENS. 425 



gives a good idea of the gradual massing of the stars from a region 

 of uniform distribution into two clusters whose stars are brighter 

 than the average of that part of the sky. 



Meteors. 



The unpredicted appearance of the occasional meteor, the sud- 

 denness with which it appears and the rapidity of its flight across the 

 sky, make it impossible to locate its path with exactness by eye 

 observations alone; though observers skilled in this class of work 

 can secure a close approximation to the path. If two such observers 

 are separated by several miles, a fair idea may be obtained of the 

 distance of the meteor and of its actual path through our atmos- 

 phere. In general, however, there is always much uncertainty 

 attached to such results. What one really sees is a more or less 

 bright point of light darting suddenly across the sky — ^the duration 

 of whose flight seldom exceeds one second of time and the image of 

 which vanishes from the brain almost as soon as it is formed. It 

 may well be imagined how difficult is the exact location of the path 

 of this fleeting point among the stars. If the meteor could have 

 left a line of light on the sky along the full extent of its course for 

 a few minutes, then one could locate its position fairly with respect 

 to the stars, and yet this would still have considerable uncertainty 

 attached to it from the fact that at best only an estimate (and no 

 measures) could be made with the naked eye of the position. 



In photographing the sky with wide angled lenses it is not an 

 uncommon thing for a meteor to take its flight across the region 

 which is being photographed. In this case when it is bright enough, 

 the meteor actually does leave a permanent path among the stars; 

 for the moving point of light affects the sensitive plate, continu- 

 ously, marking out thus a " trail " among the star images, which 

 is permanent and whose position can be measured with very great 

 accuracy. 



If a second camera, some distance from the first one, is also 

 photographing the same part of the sky the meteor trail will be re- 

 corded by both cameras and its displacement on the two plates as 

 photographed from these two points on the earth, can be determined 

 accurately and the distance and path of the meteor will become 



PROC. AMER. PHH.. SOC, XLVI. 187 CC, PRINTED MARCH 6, I908, 



