100 CELESTIAL SPECTROSCOPY. 
of magnitudes should be tabulated independently, and be regarded as 
supplementary of each other. 
The determination of the distances of the fixed stars from the small 
apparent shift of their positions, when viewed from widely separated 
positions of the earth in its orbit, is one of the most refined operations 
of the observatory. The great precision with which this minute angu- 
lar quantity—a fraction of a second only—has to be measured, is so 
delicate an operation with the ordinary micrometer, though, indeed, it 
was with this instrument that the classical observations of Sir Robert 
Ball were made, that a special instrument, in which the measures are 
made by moving the two halves of a divided object glass, known as a 
heliometer, has been pressed into this service, and quite recently, in 
the skillful hands of Dr. Gill and Dr. Elkin, has largely increased our 
knowledge in this direction. 
It is obvious that photography might be here of great service, if we 
could rely upon measurements of photographs of the same stars taken 
at suitable intervals of time. Prof. Pritchard, to whom is due the 
honor of having opened this new path, aided by his assistants, has 
proved by elaborate investigations that measures for parallax may be 
safely made upon photographie 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 de- 
pend upon the plate’s power of accumulation of a very feeble light act- 
ing continuously through an exposure of several hours, are worthy to be 
regarded 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 important 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 issuing from the bright star Maia of the Pleiades, and 
Shortly atterwards 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 Andromeda 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 inclination 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 
