I 



On Sal Nitrum and Nitro-Aerial Spirit 139 



nature of light, let us make a brief investigation of 

 the quality of the colours which are produced by light. 

 With regard to colours and the visible forms of 

 things, it is the most generally received opinion that 

 they are produced by the rays of light reflected in 

 various ways. But indeed I am not sure that this 

 way of explaining colours is quite in accordance with 

 truth. For let us suppose that a lamp is placed out- 

 side a chamber so that the rays of light, by means 

 of two apertures made in opposite walls of the chamber,, 

 may pass through the intervening space. When this 

 is done, if the eye be placed in any part of the chamber 

 except that through which the bundle of rays passes,. 

 the chamber will appear completely dark and the rays 

 of light passing through it will not be seen at all. But 

 now, let us suppose any coloured plane 3, Plate I., 

 Fig. II, to be placed obliquely to the rays passing thus 

 through the chamber ; when this is done, the plane will 

 be illuminated and by an eye placed at a will be seen 

 of some colour, suppose white ; and yet it seems that 

 the rays of light are not reflected to the said eye. For 

 if that plane is polished in the manner of a mirror and 

 suitable for the reflection of rays of light, the rays 

 falling upon it will, from its oblique position, be 

 diverted from the eye and reflected to the opposite 

 side of the chamber, towards c, to which the line of 

 reflection tends. One would naturally say that a 

 coloured surface, especially a white one, has little 

 swellings or molecules most densely crowded upon it,, 

 whose innumerable very small surfaces look in all 

 directions around ; and that the rays of light falling 

 upon these very small surfaces, which are turned 

 towards the eye wherever it is situated, are reflected 

 by them, as by so many mirrors, to the eye, and im- 

 press upon it the sense of colour. And hence it is that 



