METHODS IN MODERN PHYSICAL ASTRONOMY. 477 



mogony should take a peep through a certain key-hole. The allegory 

 reveals the great arcanum of nature, the secret, namely, that the 

 human shape divine has been evolved from the form of a fish." 



[To be concluded .] 



METHODS IN MODERN PHYSICAL ASTEONOMY* 



By M. JULES C. JANSSEN. 



IT has become an almost consecrated custom for the President of 

 this Association, instead of relating the progress which has been 

 made in all the sciences that are the objects of your investigations, to 

 treat more particularly of a single one of them, and to present a sum- 

 mary of the progress it has made. This custom appeal's an excellent 

 one to me. By it we gain in precision and authority what we lose 

 in extensiveness ; and we owe to it many masterly efforts, the impres- 

 sion of which has not yet been effaced from our minds, and which 

 cause in me a just apprehension that I may fall far short of their 

 standard. 



I shall endeavor to present to you a picture, sketched in broad 

 outline, of the progress and influence of a branch of research which 

 has played a considerable part in contemporary scientific movements, 

 and the discoveries of which have not only revolutionized our astro- 

 nomical knowledge, but have also opened new and unexpected hori- 

 zons to philosophy I mean physical astronomy. 



Physical astronomy is a wholly modern yes, as to its best parts, 

 contemporary science ; and it can be regarded as old only as concern- 

 ing its object. From the earliest times, in fact, that men began to 

 look toward the sky, and the first reflections on nature were born of 

 these fii - st observations, man has asked what is that sun whose im- 

 mense and beneficent function caused it to be designated at that early 

 period as the soul of nature. He has asked himself what is the cause 

 which lends to the moon the sweet and mysterious light that gives to 

 the nights so poetical a charm ; and afterward questions arose con- 

 cerning those brilliant points with which the celestial vault is strewed. 

 All these problems appertain to our science, but how little was man 

 then in a condition to deal with them ! Long ages of observations 

 and labor were necessary before even the corner of the veil could be 

 raised. 



This was because physical astronomy presupposes a very clear 

 knowledge of the properties of light, both as to itself and as to its 

 relations with bodies, and great perfection in the mechanical arts to 



* President's address at the French Association for the Advancement of Science, 

 La Rochelle, August, 18S2. 



