24 THOUGHTS ON NATURAL PHILOSOPHY. 



by showing us that there are regions in the heavens 

 where a faint diffused nebulosity is all that can be 

 detected by the telescope. There are other nebulae 

 in which a nucleus can be just discerned, others 

 again in which the nucleus is easily seen, and still 

 others where the nucleus is a brilliant, star-like point. 

 The transition from an object of this kind to a 

 nebulous star is very natural, while the nebulous 

 stars pass into the ordinary stars by a few graduated 

 stages. It is thus possible to enumerate a series of 

 objects beginning at one end with the most diffused 

 nebulosity and ending at the other with an ordinary 

 fixed star or group of stars. Each object in the 

 series differs but slightly from the object just before 

 it and the object just after it. It seemed to Herschel 

 that he was thus able to view the actual changes 

 by which masses of phosphorescent or glowing 

 vapour 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 the forest, by comparing the trees of various 

 ages which the forest contains at the same time. 

 In attempting to pronounce on the evidence with 

 regard to Herschel's theory, we must at once admit 

 that the transmutation of a nebula into a star has 

 never been seen. It is indeed very doubtful whether 

 any changes of a nebula have ever been seen which 

 are of the same character as the changes Herschel's 

 theory would require. It seems, however, most likely 

 that the periods of time required for such changes 

 are so stupendous that the changes accomplished in 

 a century or two are absolutely inappreciable. 



