144 ANNUAL REPORT SMITPISONIAN INSTITUTION, 1912. 



unchanged when compared with the adjacent stars. It is therefore 

 neither a planet nor a meteor. It belongs to our sun no more than 

 to the earth. Accordingly, if we admit the Copernican theor}', we 

 must attribute to this nebula colossal dimensions, far exceeding the 

 distance which separates the earth from the sun. 



Nor is the nebula of Andromeda an isolated case. Christian Huy- 

 gens made a drawuig in 1656 of a nebula in the constellation of Orion, 

 a more brilliant and more extended object the outlines of which he 

 found very difficult to trace. On one side only was it sharply de- 

 fined against the adjacent sky. Elsewhere it faded into indistingTiish- 

 able nebulosity. Does it not seem, mused Huygens, as if here we are 

 looldng upon a new world, perhaps upon the legendary empyrean ? 

 This feeble veil scarcely alters the aspect of the stars which shine 

 through it or are projected upon it. 



This somewhat summary sketch of Huygens was only vaguely con- 

 firmed by those of Picard and of Legentil who came several years later. 

 The only common trait, indeed, was the dark gulf which hollows out 

 the central part. No part of it seemed sufficiently definite for the 

 detection of possible changes. 



During the eighteenth century the number of known nebulas in- 

 creased slowly. Several, upon closer examination, proved to be clus- 

 ters of small stars. Those whose aspect remained flocculent, despite 

 all efforts to resolve them, often deceived the comet seekers, who, 

 after verification, saw their cherished hope of making the discovery 

 of a new planetoid disappear. Messier, more than once thus caught, 

 undertook to remove this cause of trouble and in 1784 published 

 a catalogue of these objects, containing nearly all the nebulae easily 

 seen above the horizon at the latitude of Paris. 



At the same time a great advance was made in England in the means 

 of observation. The musician, W. Herschel, succeeded during his 

 leisure hours in figuring and mounting telescope mirrors much greater 

 and much more perfect than had ever been made before. In the field 

 of these instruments nebulae appeared in an absolutely unexpected 

 profusion. Thus there arose a new branch of astronomy to be de- 

 veloped. Herschel set to work, aided by his sister Caroline, and with 

 remarkable perseverence, at the same time pursuing other researches, 

 catalogued, from 1786 to 1802, some 2,500 nebulae. Many of these, 

 upon closer examination, were resolved into stars. W. Herschel 

 was led to believe that all could bo so resolved and that any one of 

 these scarcely visible flocculent specks would, to an observer properly 

 situated, appear like a stellar universe as rich as that which surrounds 

 us and which is evident to our eyes tlirough the milky way. 



The work of W. Herschel was completed for the Southern Hemi- 

 sphere by his son Jolm, who transported in 1834 to the Cape of Good 

 Hope one of the best telescopes constructed at Slough by his father. 



