PROGRESS IN PHYSICS. 325 



pr()l)lein ajid iiiado tho science of s])ccti'uiu uiialy.sis siil>st:iMti:illv whiit 

 it i.s to-day. Its fundamental principles may be considered as finv and 

 comparatively simple. 



Waves of light and radiant heat originate in ether disturbances pro- 

 duced by molecular vibration and have impressed upon them all of 

 the important qualities of that vibration. Molecules of different sub- 

 stances differ in their modes of vibration, each producing a wave pecul- 

 iar to and characteristic of itself. A useful analogy may be found in 

 the fact that when one listens to the music of an orchestra without 

 seeing it, it is easy to recognize the tones that come from each of the sev- 

 eral instruments, the characteristic vil)rations of each being impressed 

 upon the waves in air which carry the sound to the ear. So delicate 

 and so sure is this impr<^ssion of vibration peculiarities that it is pos- 

 sible even to know the maker of a \nolin, for instance, b}- a character- 

 istic timbre which must have its physical expression in the sound wave. 

 The ear, more perfect than the eye. analyzes the resultant disturbance 

 into its component parts, so that each element may l)e attributed to its 

 proper source. Unaided the eye can not do this with light, but the 

 spectroscope separates the ^'arious modes of vibration which make up 

 the confused whole, so that varieties of molecular activity" are recog- 

 niza1)le. The speed at which a source of sound is approaching or 

 receding from the ear can })e ascertained by noting the rise or fall in 

 pitch due to the crowding together or stretching out of the sound 

 waves, and in the same way the motion of a luminous l)ody is known 

 from the increase or decrease of the refrangibility of the elements of 

 its spectrum. 



Indeed, had nineteenth century science accomplished nothing else 

 than the discovery of spectrum analysis, it would have marked the 

 beginning of a new epoch. By this device man is put in communica- 

 tion with every considerable body in the universe, including even the 

 invisible. The "goings on" of Sirius and Algol, of Orion and the 

 Pleiades, are reported to him across enormous stretches of millions of 

 millions of miles of space, empty save of the ethereal medium itself, 

 by this most wonderful "wireless telegraphy." And it is by the 

 vibratory motion of the invisibly small that all of this is revealed; 

 the infinitely little has enabled us to conquer the inconceivably big. 



Many important contributions to the theory and practice of spec- 

 trum analysis have been made since the time of Kirchoff and Bunsen, 

 only two or three of which can be referred to here. Instrumental 

 methods by which spectra are produced and examined have been 

 greatly perfected, and this is especially true of what is known as the 

 "diffraction grating," first used by Fraunhofer. A quarter of a cen- 

 tury ago Rutherfurd, of New York, constructed a ]-uling engine, by 

 means of which gratings on glass and spectrum metal were ruled with 

 a precision greatly exceeding what had before been possible. A few 



