ADDRESS. 35 



honour of having opened this new path, aided by his assistants, has 

 proved by elaborate investigations that measures for parallax may be 

 safely made upon photographic plates, with, of course, the advantages of 

 leisure and repetition ; and he has already by this method determined the 

 parallax for twenty-one stars with an accuracy not inferior to that of 

 values previously obtained by purely astronomical methods. 



The remarkable successes of astronomical photography, which depend 

 upon the plate's power of accumulation of a very feeble light acting 

 continuously through an exposure of several hours, are worthy to be re- 

 garded as a new revelation. The first chapter opened when, in 1880, Dr. 

 Henry Draper obtained a picture of the nebula of Orion ; but a more im- 

 portant advance was made in 1883, when Dr. Common, by his photographs, 

 brought to our knowledge details and extensions of this nebula hitherto 

 unknown. A further disclosure took place in 1885, when the Brothers 

 Henry showed for the first time in great detail the spiral nebulosity issu- 

 ing from the bright star Maia of the Pleiades, and shortly afterwards 

 nebulous streams about the other stars of this group. In 1886 Mr. 

 Roberts, by means of a photograph to which three hours' exposure had 

 been given, showed the whole background of this group to be nebulous. 

 In the following year Mr. Roberts more than doubled for us the great 

 extension of the nebular region which surrounds the trapezium in the 

 constellation of Orion. By his photographs of the great nebula in An- 

 dromeda, he has shown the true significance of the dark canals which 

 had been seen by the eye. They are in reality spaces between successive 

 rings of bright matter, which appeared nearly straight owing to the in- 

 clination in which they lie relatively to us. These bright rings surround 

 an undefined central luminous mass. I have already spoken of this 

 photograph. 



Some recent photographs by Mr. Russell show that the great rift in 

 the Milky Way in Argus, which to the eye is void of stars, is in reality 

 uniformly covered with them. Also quite recently Mr. George Hale has 

 photographed the solar prominences by means of a grating, making use 

 of the lines H and K. 



The heavens are richly but very irregularly inwrought with stars. 

 The brighter stars cluster into well-known groups upon a background 

 formed of an enlacement of streams and convoluted windings and inter- 

 twined spirals of fainter stars, which becomes richer and more intricate in 

 the irregularly rifted zone of the Milky Way. 



We, who form part of the emblazonry, can only see the design dis- 

 torted and confused ; here crowded, there scattered, at another place 

 superposed. The groupings due to our position are mixed up with those 

 which are real. 



Can we suppose that each luminous point has no other relation to 

 those near it than the accidental neighbourship of grains of sand upon 



B 2 



