Nipher — Physics During the Last Century. 117 



the apparent pitch of the sound is changed by motion of the 

 sounding body or the ear. This has enabled astronomers to 

 measure the velocity with which stars are approaching or re- 

 ceding from the earth. Double stars have been discovered 

 which no telescope can resolve. The commingled light from 

 the two stars has been separated by the spectroscope. The 

 dark lines from the light of the approaching star, are de- 

 flected towards the violet end of the spectrum, and those from 

 the receding star, are deflected in the opposite way. These 

 deflections go through periodic to-and-fro changes, corre- 

 sponding to the orbital motions around the common center of 

 gravity. When August Comte said in his Positive Philosophy 

 that while we might know the forms and distances of the 

 heavenly bodies, " we can never know anything of their 

 chemical or mineralogical condition," he really meant that 

 chemists would never be able to have samples from these 

 bodies collected and carted to Paris for analysis in test-tubes. 

 When Comte wrote these words the men were living who were 

 to analyze the stars. 



In the progress of the study of light, its identity with 

 radiant heat was established. The earlier work by Melloni, 

 Tyndall and Magnus was done with the thermo pile. The 

 more recent work of Langley has greatly increased the deli- 

 cacy of the measurements. Langley finds that only about 

 one-fifth of the energy of the solar spectrum is from visible 

 radiations. In the visible part of the spectrum, the luminous 

 and heating effects rise and fall together. The dark lines are 

 lines of lower temperature. The bolometer, which was de- 

 signed by Langley for temperature measurements of this 

 character, shows the presence of similar cold bands in the 

 invisible part of the spectrum below the red. With the latest 

 form of instrument, it is possible to measure to the millionth 

 of a degree. 



The greatest development shown in any one branch of 

 Physics, has been in electricity and magnetism. The ad- 

 vances in our understanding of the nature of magnetic and 

 electrical action have been already touched upon. Oer- 

 sted, Arago and Ampere discovered that the space around a 

 current of electricity is a magnetic field. They studied the 



