AN ADDRESS ON ASTROPHYSICS. 303 



demands a complete understanding of the action of the same forces 

 upon terrestrial matter. The astrophysicist dwells in the laboratory 

 as well as in the observatory; and. laboratory researches must supply 

 the links which connect world life and star life. 



It has not been possible for laboratory investigators to reproduce 

 stellar phenomena on a scale approaching that occurring in nature, 

 nor to duplicate conditions of temperature and pressure existing within 

 the stars; and these are unfortunate limitations. Nevertheless, many 

 successes have been achieved in this direction. The low-temperature 

 triumphs of Dewar, Olczewski and others approximate to the conditions 

 of space surrounding the stars. The electric arc and spark appear to 

 reproduce the temperatures of many stellar chromospheres and revers- 

 ing layers. The electric furnace of Moissan seems to supply tem- 

 peratures comparable with those of the photosphere, and it promises 

 to throw light upon the processes of cloud formation in the stars. 

 Investigations as to the influence of varying pressures — from almost 

 perfect vacua up to many atmospheres — as to the effects of varying 

 electrical conditions and of other factors have answered many celestial 

 questions, and introduced others equally pressing. 



Laboratory observations have established that the spectra of the 

 elements are not the same under all circumstances. We formerly 

 thought it remarkable that nitrogen should have two or three char- 

 acteristic spectra, or that a metal should have a spark spectrum and 

 an arc spectrum. We are now confronted with the potent fact that 

 an element may have a variety of spectra, depending upon the nature 

 and the intensity of the forces employed in rendering it luminous. 

 But for most cases these involve only moderate variations in the 

 relative intensities of spectral lines. The complications which threaten 

 to result therefrom are more apparent than real. The multiplicity of 

 spectral reactions promises to be a powerful aid to analysis, by sup- 

 plying a more exact key to the conditions in the celestial light source 

 which produce the observed effects. 



For many years following the application of the spectroscope to 

 celestial problems it was supposed that a continuous spectrum must 

 indicate incandescent solid or liquid, matter. The situation is not 

 so simple as this. Some gases radiating under high pressures give 

 spectra apparently continuous. 



The effect of increasing temperature conditions on certain spectra 

 has long been well known. Certain lines are enhanced in relative 

 brilliancy when we pass from the temperature of the arc to that of the 

 high-tension spark, and vice versa; but it seems certain that, within 

 measurable limits, the positions of the lines do not change under this 

 influence. 



Humphreys and Mohler have proved that the spectral lines are 

 shifted by pressure; — toward the red with increasing pressure in the 



