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shows the structure and details of the central nebulosity with greater 
clearness than the first. ‘The second shows vastly more extensive 
nebulosity than the first, but the central part is too dense on the 
negative to print on the paper enlargement. ‘The stars and all 
details of the central nebulosity are, nevertheless, clearly visible on 
the negative. These two photographs, when correlated with each 
other, show the great nebula more completely and truly than it was 
previously known; and, though many drawings have been made and 
ably discussed by Prof. Holden in his elaborate monograph on the 
Orion nebula, they only show how utterly untrustworthy eye obser- 
vations are. ‘The first photograph of this object was obtained by 
Dr. Draper, in 1880, with an eleven-inch refractor, his best one 
being obtained in March, 1882, with an exposure of 137 minutes. 
The next advance was by Dr. Common, in 1883, with his three- 
foot reflector and an exposure of 37 minutes. ‘This, in turn, has 
been much distanced by the present photograph, which shows an 
enormous extension of nebulosity and much delicate detail not 
before seen. 
On the photograph of the great nebula in Andromeda (PI. xiil.), 
the sky area covered is 1 deg. 54 m. by 1 deg. 38 m., on a scale of 
24 seconds of arc to 1 millimeter, and was taken with the twenty- 
inch reflector, December 29, 1888, with an exposure of four hours. 
The nebula is one of the largest in the heavens, and has been known 
ever since the invention of the telescope as a long, oval nebulosity, 
ill-defined at the margin. Bond, in 1847, and Trouvelot, later, 
with the fifteen-inch Harvard refractor, detected two large, longi- 
tudinal rifts on one side of it. No advance was made beyond this 
until my photograph, taken on October ro, 1887, revealed its true 
form for the first time. The nebula is shown to be symmetrically 
oval and encompassed by elliptical rings, separated by dark divi- 
sions extending completely around it. ‘There are a great many stars 
involved, apparently, in the nebula, which the photograph shows in 
their true relative positions, together with the structure and details 
of the nebulosity. 
In conclusion, I shall only be uttering a truism when I say that 
we are yet only at the threshold of knowledge of the stellar universe, 
though the progress made during the past ten years encourages us 
to hope that ere the Two Hundredth Anniversary of the Philosoph- 
ical Society of Philadelphia shall be held much will be known con- 
cerning the movements of the solar system in space, the general drift 
