A CHAPTER ON LIGHT AND COLOUR. 113 



Red flames were seen on the border of the ecHpsed sun, some of 

 which changed their form, while others remained apparently sta- 

 tionary. The study of these forms progressed but slowly, owing 

 to the rarity of the opportunity afforded by a total solar ecHpse. 



In 1868, however, it was found possible to observe them in 

 full daylight, owing to the fact that their light does not give a con- 

 tinuous spectrum, but only a few bright lines. These lines are 

 not weakened by great dispersion, while the bright background of 

 sunlight can be rendered more and more faint by employing larger 

 and larger dispersive powers, and by the use of a suitable absorb- 

 ing medium. If, now, instead of allowing the light to pass 

 through a slit, the margin of the Sun's disc is* viewed directly 

 through the spectroscope, the bright lines produced by the slit will 

 be replaced by a succession of images of these jets of flame. By 

 this means the Prominences are now the subject of continuous 

 study. They consist very largely of hydrogen, and some of them 

 extend to 100,000 miles from the sun's surface, being, on occa- 

 sion, shot forth with velocities reaching to 100 miles per second, 

 or even beyond. 



26. — The General Structure of the Eye is, briefly, as follows : 



The eyeball is approximately spherical, fitting into a bony 

 socket, in which it is free to turn in all directions with but little 

 friction. It has a tough covering called the Sclerotic Membrane, 

 mostly white and opaque, but in front it is transparent, forming 

 the Cornea, This part is shghtly more protuberant than the rest 

 of the eyeball. The body of the eye is divided into two parts, 

 the anterior of whicli is filled with the so-called Aqueous Humour, 

 the posterior with the Vitreous Humour. These are separated 

 by the Crystalline Lens. The incident light is partially stopped 

 by an opaque screen, the Iris, in the centre of which is the Pupil, 

 which is circular in man, though of difi'erent forms in some other 

 animals. This part of the eye serves simply to bring the light 

 from external objects to a focus on the retina. At the back of the 

 eye, within the sclerotic membrane, is another coating called the 

 Choroid, and between it and the vitreous humour is the Retina. 

 Over the retina spreads a fine network of nerve fibres, uniting in 

 the Optic Nerve, which runs into the brain. 



This layer of nerve fibres is followed by several other layers, 



