514 THE EYE AS OPTICAL INSTRUMENT. 



pared, in powerful spectroscopes, with those of the bright 

 lines in the spectra of certain terrestrial substances 

 (metals), it is seen that each of the bright lines of these 

 particular substances coincides exactly with a dark solar 

 line ; so that if the apparatus be arranged in such a 

 way that the Sun's spectrum and one of these metallic 

 spectra appear one below the other., in the field of the 

 telescope, which in large instruments replaces the simple 

 aperture of our spectroscope, the bright lines of the 

 metal are all seen to be continuous with dark solar 

 lines. In the case of metallic iron alone, which may 

 be vaporised by the electric spark, more than 80 bright 

 lines appear in the spectrum of its vapour, all of which 

 exactly coincide with 80 dark lines in the solar spec- 

 trum. We may therefore reasonably conclude that our 

 views of the constitution of the Sun are correct, and 

 that, among other substances, the gaseous envelope of 

 the Sun contains vapour of iron. 



The same methods of observation and reasoning 

 apply to the determination of the chemical and physical 

 constitution of the atmospheres or gaseous envelopes of 

 fixed stars and other celestial bodies, and spectrum 

 analysis has indeed led. in recent times, to most im- 

 portant additions to our knowledge of fixed stars, 

 nebulae, and comets. 



42. The .Eye. Vision. Opticallnstruments. The eye 

 has some resemblance, in principle as well as in the 

 arrangement of its parts, with a camera obscura. 

 Fig. 282 represents a horizontal section of the right 

 eyeball. It is nearly spherical in shape, and is com- 

 posed, in the first place, of a tough firm coating 

 consisting of fibrous tissue, the greater part of which 



