i84 SCIENCE PROGRESS 



lines and detecting dissimilarity in the spectra of a few of the 

 brighter stars. Forty years later Donati, doubtless incited by 

 KirchhofTs work, had utilised a large single lens with an 

 aperture of sixteen inches, belonging to the Florence Museum, 

 to gather the light of the brighter stars for examination with a 

 single prism. Donati passed the light collected by his lens 

 through a second lens of short focus and then through a single 

 prism, and viewed the spectrum with a short telescope. He 

 used a diaphragm with wide opening for the purpose of setting 

 each star image in the axis of the collimating lens but did not 

 employ any slit. With this apparatus Donati studied the spectra 

 of about fifteen bright stars and published his observations in 

 1862. He found differences, some of which may be recognised 

 now as really attributable to differences in the nature of the 

 stars, whilst others are no less clearly due to imperfections 

 in his method. 



In both Fraunhofer's and Donati's methods there lies a great 

 advantage, and also a great disadvantage. The advantage is 

 that all the light collected by the object-glass is utilised and 

 relatively brilliant spectra result. The disadvantage is that they 

 were unable to get trustworthy fiducial marks, so that com- 

 parisons of measurements were in general nugator}^ 



Huggins and Miller adopted the plan of attaching to the 

 equatorial a spectroscope equipped with a slit, in such a manner 

 that the image of the star could be brought upon the slit and 

 that the light passed down the tube of the collimator and so 

 through the prisms and the viewing telescope. They also 

 arranged small reflecting prisms over parts of the slit, so that 

 the light from a terrestrial source, such as a coloured flame or an 

 electric spark, could be passed from a lateral direction into the 

 slit. In this manner they were enabled to see the spectrum of 

 a star and the spectrum of the terrestrial source simultaneously. 

 It was delicate, laborious work, rendered all the more difficult 

 by the tremulous motion of the star upon the narrow slit (not 

 more than shir^h or j^iyth of an inch wide). But with unremitting 

 patience and in spite of constant interruption of clouds and bad 

 weather, they succeeded in making measurements which con- 

 clusively proved that the stars contain materials similar to those 

 found on the earth and in the Sun. 



Such conclusions depended on their employment of the 

 fixed spectra of terrestrial sources as intermediary standards 



