THE SUN. 49 



He suspected that they might be an optical 

 effect, depending on the quality of the glass 

 used, and he tried different prisms, but the 

 lines were still to be seen. Then he turned 

 his prism to bright clouds to see if they were 

 visible in reflected sunlight, and he found that 

 they were. He examined the Moon and again 

 perceived them, as moonlight is merely reflected 

 sunlight ; and they were also conspicuous in the 

 spectra of the planets. It was thus proved that 

 these lines were characteristic of sunlight, whether 

 direct or reflected. It was, however, still possible 

 that they might be caused by the passage of the 

 rays of light from the celestial bodies through the 

 Earth's atmosphere. In order to test this theory, 

 Fraunhofer examined the spectra of the brighter 

 stars. He found that the lines visible in the 

 solar spectrum were not to be seen in the spectra 

 of the stars, thus proving that the lines were not 

 merely an atmospheric effect. Each star, Fraun- 

 hofer observed, had a different spectrum from 

 both the Sun and from other stars. These 

 spectra were also characterised by numerous 

 dark lines, much fainter than those in the solar 

 spectrum. 



Although he ascertained the existence of the 

 dark lines in the Sun's spectrum, Fraunhofer 

 never really found out what they represented. 



