WHITE SUBSTANCES. 231 



ters on spectres and ghosts, that the image seen in the frag- 

 mentary surfaces would be divided, and the portions va- 

 riously dispersed; for the broken surfaces of the glass, 

 being inclined to each other, would form different angles 

 with the rays incident upon them, and would reflect them 

 differently, and thus a mass of confused and disjointed 

 gloamings would be the result. 



If, instead of being broken into fragments of sensible 

 size, the glass were to be ground into a fine powder, the 

 particles would still reflect the light falling upon them, but 

 the rays would be mingled in inextricable confusion, and, 

 instead of producing any distinct images in the eye, or 

 even distinct portions of images, they would only produce 

 the impression of a general white light. 



This is supposed to be the manner in which the sensa- 

 tion comes to us from any white substance. The compo- 

 sition of the surface is such that it takes up the light that 

 falls upon it, and reflects it to our eyes in a confused med- 

 ley of beams that can form no image on the retina, but 

 only produce a general luminous effect. 



If, however, we contrive by artificial means to smooth 

 and polish any white surface, as that of marble, for in- 

 stance, we can impart to it the power of faintly and indis- 

 tinctly reflecting an image that is, by the process of pol- 

 ishing, we form among the particles so many faces lying in 

 the same plane, that by their combined effect they throw a 

 sufficient number of rays, coming from any object near, in 

 a regular manner to the eye, to form an image distinct, 

 perhaps, in its general features, but faint, on account of 

 these rays being mingled with others coming in confusion 

 from the particles which are not in the same plane. 



This, then, is the explanation of the manner in which the 

 sensation of whiteness is formed in our organs of vision. 

 A surface that appears white reflects the light that falls 



