300 IN STARRY REALMS. 



to a point on the retina, and if the image there produced 

 be sufficiently intense the sense of vision is excited, and 

 the star is seen. If, however, the star be not perceived 

 at the first glance, there is but little object in prolonging 

 the gaze. It is true that expert practical astronomers 

 know that a star which they fail to see when directly 

 looking at it can sometimes be " glimpsed " when the eye 

 is moved slightly away ; the explanation apparently being 

 that some fresher and more sensitive part of the retina is 

 by this act brought into use. But by merely steadily 

 staring at a faint star which is not bright enough to be 

 detected at the first glance, there is little success to be 

 expected. The fact is that the retina can only retain 

 an impression for a small time perhaps about one-seventh 

 of a second consequently there is no cumulative effect 

 of the luminous impression to be obtained by prolonged 

 watching. 



But the case is very different when we place in the 

 focus of the telescope a highly sensitive photographic 

 plate, and permit the instrument to depict thereon an 

 image of the star. The vibrations of the rays of light throw 

 themselves assiduously on the plate, and steadily apply 

 themselves to the task of shaking asunder the molecules of 

 silver salts in the gelatine film. Just as the waves of 

 ocean by incessantly beating against a shore will gradually 

 wear away the mightiest cliffs of the toughest rock, so the 

 innumerable millions of waves of light persistently im- 

 pinging upon a single point of the plate will at length 

 effect the necessary decomposition, and so engrave the 

 image of the star. It will be obvious that this process 

 will be the more complete the longer the exposure which 



