ASTRONOMY OF EIGHTEENTH CENTURY, 271 



FIG. 127. THE ZODIACAL LIGHT. 



by the name of the Zodiacal Light. It should be mentioned that the 

 zodiacal light is also visible before sunrise. The matter of the zodiacal 

 light, whatever its nature, is sufficiently attenuated to allow the light of 

 the smallest stars to pass through it without perceptible diminution. 

 Laplace supposed the zodiacal light to be a residual portion of the 

 original nebulous mass of the solar system, and if its nebulous nature 

 should be satisfactorily proved by observation, Laplace's hypothesis will 

 receive additional confirmation. The hypothesis, not being founded 

 on observation or calculation, was put forward by Laplace himself with 

 much diffidence. But it unites so many apparently unconnected facts 

 by a few simple and general laws, and those, too, laws known to.be in 

 action, that it has every mark of probability to recommend it. This 

 explanation of the origin of the solar system involves no invention of 

 a new agency, and no assumption for the occasion of special laws to 

 suit the case. The hypothetical part consists only in the assumption 

 of a certain condition of things at a remote epoch, namely, the exist- 

 ence of all the matter of the solar system in a vaporous state. The rest 

 is deduced from this as the effects of known laws ; and, if the a priori 

 evidence in favour of the hypothesis is weak, it is weak only in so far as 

 we are unacquainted with all the conditions under which the known 

 laws operated in distant time. Again, Laplace's hypothesis transcends 

 every other that had been put forward to explain the origin of our 



