894 



SCIENCE 



[N. S. Vol. XXXIV. No. 887 



order to prevent this overlapping, two im- 

 portant modifications must be made in 

 Newton's arrangement. First the light 

 must be allowed to pass through a very 

 narrow aperture, and second, a sharp 

 image of this aperture must be formed by 

 a lens or mirror. 



The first improvement was introduced 

 by Wollaston in 1802, who writes: 



If a beam of daylight be admitted into a dark 

 room by a crevice 1/20 of an inch broad and re- 

 ceived by the eye at a distance of 10 or 12 feet 

 through a prism of flint glass held near the eye, 

 the beam is seen to be separated into the four 

 colors only, red, yellowish green, blue and violet. 

 . . . The line that bounds the red side of the spec- 

 trum is somewhat confused. . . . The line between 

 the red and green ... is perfectly distinct; so 

 also are the two limits of the violet. There are 

 other distinct lines (in the green and blue . . .). 



The second improvement was effected by 

 Fraunhofer, 1814, and by observing the 

 light which fell from such a narrow aper- 

 ture upon a prism by means of a telescope 

 he discovered upward of 750 dark lines in 

 the solar spectrum, and mapped their 

 position and general character. 



In recognition of the enormous import- 

 ance of this discovery, these lines are al- 

 ways known as the Fraunhofer lines. 



A minor inconvenience in Fraunhofer 's 

 arrangement lay in the fact that the slit 

 source had to be at a considerable distance 

 from the telescope; and this was obviated 

 in the apparatus of Bunsen and Kirchhoff, 

 1860, which is essentially the same as the 

 modern spectroscope of to-day; consisting 

 of a slit and collimator, prism and observ- 

 ing (or photographic) telescope. 



And on this beautifully simple device 

 rests practically the whole science of spec- 

 troscopy, with all its wonderful applica- 

 tions and all the astonishing revelations of 

 the structure and motions of the sidereal 

 universe, and of the constitution of the 

 atoms of matter of which it consists — nay 



even of the electrons of which these atoms 

 are built! 



Without the telescope it is evident that 

 the science of spectroscopy would be as lim- 

 ited in its field as was the science of astron- 

 omy without the telescope. It is interest- 

 ing indeed to compare the progress of the 

 two sciences as dependent on the successive 

 improvements in the two instruments. 



Without the telescope nothing could be 

 discovered concerning the heavenly bodies 

 (with the exception of a few of the more 

 evident features of the sun, the moon and 

 the comets) except the brightness and 

 places of the stars, and the motion of the 

 planets — and even these could at best be 

 very roughly determined (say to within 

 one part in five thousand or something 

 over a half minute of arc). Without the 

 telescope spectroscopy would also have been 

 limited to observations of general differ- 

 ences in character of radiations and ab- 

 sorptions, and a rough determination of 

 the position of the spectral lines, with a 

 probable error of this same order of magni- 

 tude. 



In fact the resolving power of the eye is 

 measured by the number of light waves in 

 its diameter of the pupil, about 5,000, and 

 if a double star (or a double spectral line) 

 presents a smaller angle than 1/5,000 it is 

 not ' ' resolved. ' ' The resolving power of a 

 telescope with a one inch objective would 

 be about 100,000 ; so that details of the solar 

 and lunar surfaces and of planets, nebulae 

 and of double stars and star groups can be 

 distinguished whose angular distance is of 

 the order of 1/100,000. The discs of the 

 planets, the rings of Sat^^rn, the moons of 

 Jupiter, and some star groups and clusters, 

 begin to be distinguishable. Our largest 

 telescopes have a resolving power as high 

 as 2,000,000, corresponding to a limit of 

 separation of one tenth of a second. 



But in order to realize the fuU benefit 



