Mars and the Minor Planets 245 



FIG. 147. Photograph showing the trails of two minor planets. The 

 telescope was set to move with the stars, and the images of the planets 

 show as streaks on the plate. 



the discovery of new ones has been made easier by pho- 

 tography. In a large photographic telescope a plate is ex- 

 posed for several hours. If the telescope were motionless 

 it would keep pointing farther east among the stars as 

 the earth rotates, but clockwork is arranged to coun- 

 teract this motion, causing the instrument to move to 

 the west as fast as the stars do. When the plate is 

 developed the stars appear as points, but the planets as 

 streaks, for they moved while their pictures were being 

 taken. Of late years astronomers have reversed this by 

 making the telescope's motion conform nearly to that 

 of the average minor planet, so that the stars, instead 

 of the planet, show on the photograph as streaks. 



