240 THE POPULAR SCIENCE MONTHLY. 



fixed star or group of stars. Each object in the series differs but 

 slightly from the object just before it and just after it. It seemed to 

 Herschel that he was thus able to view the actual changes by which 

 masses of phosphorescent or glowing vapor became actually condensed 

 down into stars. The condensation of a nebula could be followed in 

 the same manner as we can study the growth of the trees in a forest 

 by comparing the trees of various ages which the forest contains at the 

 same time. In attempting to pronounce upon the positive evidence 

 available in the discussion of Herschel's theory, we encounter a well- 

 known difficulty. To establish this theory, it would be necessary to 

 watch the actual condensation of one single nebula from the primitive 

 gaseous condition down to the stellar points. It may easily be con- 

 ceived that such a process would require a vast lapse of time, perhaps 

 enormously greater than the period between the invention of the tele- 

 scope and the present moment. It may at all events be confidently 

 asserted that the condensation of a nebula into a star is a process 

 which has never been witnessed. Whether any stages in that process 

 can be said to have been witnessed is a different matter, on which it is 

 not easy to speak with precision. Drawings of the same nebula, made 

 at different dates, often exhibit great discrepancies. In comparing 

 these drawings, it must be remembered that a nebula is an object usu- 

 ally devoid of distinct outline, and varying greatly in appearance with 

 different telescopic apertures. Take, for instance, the very splendid 

 nebula in Orion, which is one of the most glorious objects that can be 

 seen in a telescope. There can be no doubt that the drawings made 

 at different times do exhibit most marked differences. Indeed, the 

 differences are sometimes so great that it is hard to believe that the 

 same object is depicted. It is well to look also at drawings made of 

 the same object at the same time, but by different observers and with 

 different telescopes. Where we find contemporary drawings at va- 

 riance and thej r are often widely at variance it seems hard to draw 

 any conclusion from drawings as to the presence or the absence of 

 change in the shape of the nebula. 



There are, however, good grounds for believing that nebula? really 

 do undergo some changes, at least as regards brightness ; but whether 

 these changes are such as Herschel's theory would seem to require is 

 quite another question. Perhaps the best authenticated instance is 

 that of the variable nebula in the constellation of Taurus, discovered 

 by Mr. Hind in 1852. At the time of its discovery this object was a 

 small nebula about one minute in diameter, with a central condensa- 

 tion of light. D'Arrest, the distinguished astronomer of Copenhagen, 

 found, in 1861, that this nebula had vanished. On the 29th of De- 

 cember, 1861, the nebula was again seen in the powerful refractor at 

 Pulkova, but, on December 12, 18G3, Mr. Hind failed to detect the 

 nebula with the telescope by which it had been originally discovered. 

 This instrument had, however, only half the aperture of the Pulkova 



