40 PRESIDENTIAL ADDRESS — SECTIOX A. 



But optics owes yet some other developments to astronomy. 



It was, indeed, optics with its spectroscopic methods which 

 assisted astronomers in the search for the physical and chemical 

 constitutions of celestial bodies ; but, when stellar spectra were 

 compared with terrestrial ones, some differences appeared, the 

 origin of which it became necessary to ascertain. This gave 

 physicists some important data on the effect of pressure and 

 temperature on the features of spectra. 



In terrestrial laboratories, it is not possible to get such high 

 temperatures and pressures as we find in the stars. Stars are the 

 great laboratories from which the astronomer takes the 

 observations that are useful to the physicist. 



The determination of radial velocities of the stars, which 

 seems to be a problem that interests only the astronomer, 

 constitutes a strong aid to the physicist, because it means strong 

 experimental support of the wave theories of light. Had the 

 astronomer not been able to detect such velocities by the spectro- 

 scopic method, such an experimental confirmation would not have 

 been obtained, because on earth we have not velocities sufficiently 

 great to render the phenomenon measurable. 



Not only in the restricted field of optics can astronomy assist 

 Science. As you know, we have nowadays a fair knowledge of 

 the chemical constitution of the stars, but this constitution differs 

 according to the class of each star. 



There are elements that are only observed in some states of 

 stellar evolution. It is for astronomy to say if there are elements 

 that actually exist, but are not observable, or if these elements 

 have already disappeared or will appear later, by transformation 

 of others, as may be the way of stellar evolution. As to stellar 

 evolution, our knowledge is somewhat hypothetical. 



The astronomer among stars has been compared to the 

 traveller among the trees in a forest, who, although he does not 

 see the trees growing, can, however, know their evolution as he 

 sees them in all their ages. The comparison is not a very good 

 one, because the traveller knows, by former experience, that 

 trees born small, grow up and reproduce themselves. The 

 astronomer cannot have a similar experience about the stars, 

 because life is too short. 



I will end this address with the most recent instance of the 

 assistance of astronomy to physics. 



The existence of a medium, assumed in both the light 

 ■theories of Fresnel and of Maxwell (although with different 

 properties), gave physicists the wish to determine the velocity of 

 the earth relative to it. 



The discovery of the aberration of light by the astronomical 

 observations of Bradley in 1728 showed that such a medium is not 

 drawn by the earth in its motion. 



Due to similarity of luminous and sonorous phenomena and 

 theories, physicists believed it would be possible to detect an 



