78 MR. J. SMITH ON THE ORIGIN OF COLOUR 



in gaslight^ then, a sheet of white paper could be used in 

 place of a white cloud, and the different effects produced 

 by changing the angle of the incident light, that is by 

 reflecting on the disc either more light or less light as 

 might be thought proper, became most instructive, much 

 more so than with the light from the white cloud, which is 

 less manageable. 



119. The method of proceeding was as follows: having 

 placed the disc on the wheel, and made it revolve before a 

 white cloud, or above a sheet of white paper highly illumi- 

 nated, the holes cut in the disc permitted the light from 

 the cloud or the paper to pass through them as the disc 

 revolved. The light from the cloud or paper being inter- 

 cepted at intervals by the opaque parts of the disc, colour 

 was produced on the principles already explained. The 

 change of colour, which results from a change in the quan- 

 tity of light, caused by a change of the inclination of the 

 paper to the light incident upon it as well as the disc, 

 explains many natural phenomena. 



I may mention that a sheet of ground glass hung up 

 before a window may be used instead of the cloud or 

 paper : semi-discs may be employed before such a sheet 

 of glass. 



In many cases also the light reflected from a mirror 

 answered better than the light of the sky. When the 

 ground is covered with snow these experiments succeed 

 very well. 



HOEIZONTAL MOTION. 



3ed Class of Experiments. 



Solar Spectra by Reflection, or Coloured Shadows. 

 120. In the experiments which I am now about to de- 

 scribe I made use of the apparatus employed in the second 



