72 Nebulce and Nebular Hypotheses, [Sess. 



but with all its power, one thing it cannot do is to convey 

 to us any adequate idea of the form of a nebula. 



But with the advent of photography our knowledge of these 

 objects has advanced by leaps and bounds. For nebulae are 

 so exceedingly faint that very few of them are visible at all 

 to the keenest human eye, even when fortified by a powerful 

 telescope, and these few can just be glimpsed and no more. 

 But by merely exposing a photographic plate to their light 

 for a long enough time, a perfect picture can be obtained 

 showing all their structural details. Not that the plate is 

 more sensitive than the eye ; but the effect of light on the 

 eye is momentary, while on the plate it is cumulative. How- 

 ever feeble the rays may be, their persistent battering upon 

 the same portion of the plate at last tells upon the film, and 

 engraves the image of the object. Moreover, the quality of 

 nebular light is not such as readily affects our sense of vision, 

 while it is particularly powerful photographically. Hence the 

 most lynx-eyed observer can never hope to compete with the 

 camera in delineating the forms of nebulse. And a third 

 advantage of photography is, that with a large telescope only 

 a small part of the object can be seen at a time ; while on 

 the plate the whole can be depicted in all its marvellous 

 entirety. 



In the majority of cases nebulae are apparently only the 

 brightest portions of a mysterious substance which seems to 

 cover practically the whole sky like a gauzy veil, and which 

 is rent and torn in one region while it is gathered into folds 

 and puckers in another. They exhibit the most fantastic 

 variety of form, and to telescopic scrutiny they seemed to 

 borrow the similitude of all sorts of terrestrial objects. 

 Nebulae were found in the semblance of fans, of rings, of 

 brushes ; some were elliptical and others circular ; some were 

 triangular and others quadrilateral; one resembled an owl, 

 another a crab, a third a dumb-bell, and a fourth a fish's 

 mouth. But with the aid of photography such fanciful 

 imageries have disappeared; and the chief result of later 

 investigations is that " structure has become increasingly mani- 

 fest among all classes of nebulae. Structure not of the finished 

 kind, but indicating with great probability the advance of 

 formative processes on an enormous scale, both as regards 



