220 RECENT PEOGKESS OF THE THEORY OF VISION. 



is most apparent in the blue and violet rays of the solar 

 spectrum ; for there comes in the phenomenon of fluo- 

 rescence 1 to increase it. 



In fact, although the crystalline lens looks so beauti- 

 fully clear when taken out of the eye of an animal just 

 killed, it is far from optically uniform in structure. It 

 is possible to see the shadows and dark spots within the 

 eye (the so-called ' entoptic objects ') by looking at an 

 extensive bright surface the clear sky, for instance 

 through a very narrow opening. And these shadows are 

 chiefly due to the fibres and spots in the lens. 



There are also a number of minute fibres, corpuscles 

 and folds of membrane, which float in the vitreous 

 humour, and are seen when they come close in front 

 of the retina, even under the ordinary conditions of 

 vision. They are then called muscce volitantes, because 

 when the observer tries to look 2 at them, they naturally move 

 with the movement of the eye. They seem continually 

 to flit away from the point of vision, and thus look like 

 flying insects. These objects are present in everyone's 

 eyes, and usually float in the highest part of the globe of 

 the eye, out of the field of vision, whence on any sudden 

 movement of the eye they are dislodged and swim freely 

 in the vitreous humour. They may occasionally pass in 

 front of the central pit, and so impair sight. It is a 



1 This term is given to the property which certain substances possess of 

 becoming for a time faintly luminous as long as they receive violet and 

 blue light. The bluish tint of a solution of quinine, and the green colour 

 of uranium glass, depend on this property. The fluorescence of the cornea 

 and crystalline lens appears to depend upon the presence in their tissue of 

 a very small quantity of a substance like quinine. For the physiologist 

 this property is most valuable, for by its aid he can see the lens in a living 

 eye by throwing on it a concentrated beam of blue light, and thus ascertain 

 that it is placed close behind the iris, not separated by a large ' posterior 

 chamber,' as was long supposed. But for seeing, the fluorescence of the 

 cornea and lens is simply disadvantageous. 



8 Vide supra, p. 213. 



