OP ARTS AND SCIENCES. 509 



had honored the Academy by coming from his distant home 

 to receive the medals in person. Before presenting the 

 medals, he asked the indulgence of the Academy while he 

 gave a hasty sketch of the work which had won for Professor 

 Langley the award of the Rumford Premium. 



The progress of science, by enlarging our ideas of the complex 

 operations of nature, has introduced a new phraseology for describ- 

 ing them which would be unintelligible to men of a former genera- 

 tion. The chemistry of the sun and astro-physics would have had no 

 meaning for them. Formerly, the two great divisions of Astronomy 

 were designated as Practical and Physical ; Physical Astronomy being 

 limited to the mathematical development of the law of gravitation. 

 The application of physical instruments, such as the thermo-electric 

 thermometer, the spectroscope, the phosphoriscope, and the photo- 

 graphic camera, — all now common servants of astronomy, — to the 

 analysis of the luminous, calorific, and chemical radiations from the 

 sun, has given a wider meaning to Physical Astronomy ; so that what 

 was once understood by that phrase may be more properly called 

 mathematical or mechanical astronomy, — the Mecanique Celeste of 

 Laplace. 



Long familiarity with the facts of gravitation has made science in- 

 sensible to its mysterious mode of action ; although the labors of Fara- 

 day and Maxwell make it improbable that it acts independently of a 

 material medium between the attracting bodies. On the other hand, 

 the radiations of light and heat have been brought completely under 

 the control of the theory of undulations. While the intensity of light 

 depends partly on the physiological adaptation of the eye, radiant 

 energy can be revealed to physical instruments capable of measuring 

 the amount of heat which is emitted. Thermo-electricity was discov- 

 ered by Seebeck, of Berlin, in 1821. In 1834, Melloni constructed 

 his highly sensitive thermo-electric thermometer (called the thermo- 

 multiplier), by means of which he proved that the laws of reflection, 

 refraction, dispersion, absorption, interference, and polarization apply 

 to heat as well as to light, and that the theory of undulations is as 

 true of one as of the other. 



While astronomy was winning new victories every year in studying 

 the motions of the heavenly bodies, there seemed to be no hope that it 

 could ever detect the motion of a star in the line of vision, as its disk 

 is immeasurably small and a spurious one. Great interest was felt, 

 therefore, in the attempts of Huggins and others to solve this problem 



