344 ADDRESS ON PRESENTING THE GOLD MEDAL OF THE [45 



these objects exhibit could not result "if the nebulous matter had no other 

 quality than that of shining, or had so little solidity as to be perfectly 

 transparent." 



He also suggests that comets may be composed of nebulous matter in 



a highly condensed state, and that the faint nebulous branches which are 



often seen appended to a nucleus may be similar to the Zodiacal Light 

 in relation to our Sun. 



In the same paper he finds reason to conclude that the distance of 

 the faintest part of the great nebula in Orion probably does not exceed 

 that of stars of the 7th or 8th magnitude, but may be much less, perhaps 

 even not exceeding the distance of stars of the 2nd or 3rd order, and 

 consequently that "the most luminous appearance of this nebula must be 

 supposed to be still nearer to us." 



These views of Sir William Herschel respecting the gradual formation 

 and growth of stars by the condensation of nebulous matter were still 

 further confirmed and developed in his paper in the Philosophical Trans- 

 actions for 1814. 



Sir John Herschel's graphic description of the two Nubeculae, or Magel- 

 lanic clouds, likewise clearly shews that irresolvable nebulae, resolvable nebulae, 

 and clusters of stars represent luminous matter in different conditions, but 

 not necessarily at very different distances from us. 



The direct measurement of the distance of a nebula by determining 

 its annual parallax must be regarded as nearly hopeless. The nearest 

 known fixed star has a parallax of scarcely one second. Now the error to 

 which we are liable in the determination of the place of a nebula, although, 

 as we shall see, it may under favourable circumstances be made much smaller 

 than has been commonly supposed, still considerably exceeds one second. 

 Hence, unless a nebula were much nearer to us than the nearest fixed 

 star, there would be no chance of our being able to determine its parallax. 



There is one method, however, by which we may expect ultimately to 

 throw great light on the mutual relations of the nebular and sidereal 

 systems, and on their relative distances from us : I mean by the study of 

 their proper motions. Of course, no definite conclusion respecting the distance 

 of an individual nebula could be drawn from the observation of its proper 

 motion. For a nebula comparatively near to us might still have a very 

 small proper motion, simply because its motion in space was nearly equal 



