120 ON THE RELATION OF OPTICS TO PAINTING. 



part of the field of view are seen more distinctly. If 

 the flame is partly screened by a ruler, this appears 

 jagged where the flame projects beyond it. The lu- 

 minosity in the neighbourhood of the flame is so in- 

 tense, that its brightness can scarcely be distinguished 

 from that of the flame itself; as is the case with all 

 bright objects, the flame appears magnified, and as if 

 spreading over towards the adjacent dark objects. 



The cause of this phenomenon is quite similar to 

 that of aerial perspective. It is due to a diffusion of 

 light which arises from the passage of light through 

 dull media, excepting that for the phenomena of aerial 

 perspective the turbidity is to be sought in the air in 

 front of the eye, while for true phenomena of irradiation 

 it is to be sought in the transparent media of the eye. 

 When even the healthiest human eye is examined by 

 powerful light, the best being a pencil of sunlight 

 concentrated on the side by a condensing lens, it is 

 seen that the sclerotica and crystalline lens are not per- 

 fectly clear. If strongly illuminated, they both appear 

 whitish and as if rendered turbid by a fine mist. Both 

 are, in fact, tissues of fibrous structure, and are not 

 therefore so homogeneous as a pure liquid or a pure crys- 

 tal. Every inequality, however small, in the structure 

 of a transparent body can, however, reflect some of the 

 incident light that is, can diffuse it in all directions. 1 

 1 I disregard here the view that irradiation in the eye depends on 



