AND THE THEORY OF LIGHT. 77 



paper was made to represent the white ray of light^ the 

 black might be painted on the disc, or the spaces cut out 

 as fancy dictated, for the open spaces appeared black from 

 reflecting no light. In this class of experiments, only discs 

 with open spaces could be used, as the white of the sky 

 was here made to represent the white ray, and was there- 

 fore transmitted through open spaces. The whole disc, 

 after the parts were cut out which were to allow the pas- 

 sage of light, was sometimes painted black, but the shadow 

 cast by the paper itself, when held between the light and 

 the eve, I found to be alone sufiicient to produce colour. 



Half discs could seldom be used in these experiments. 

 In my former experiments it was found that to obtain 

 distinct colours it was necessary to destroy a great portion 

 of the incident light; consequently half discs were pre- 

 ferable to whole ones. In this class of experiments, in 

 order to destroy a necessary portion of the incident light, 

 whole discs were employed in preference to half discs. 

 For not only the light and shade on the discs but the 

 method of using them was different. In the former 

 method the parts cut out were dark; in this they were 

 white. When employing the former method the light was 

 reflected from the disc ; in this the disc was held so as to 

 be made to revolve between the eye and the light, and 

 the light was transmitted. 



I at first experimented with these discs by making 

 them revolve between my eye and a white cloud, but as 

 there were seldom white clouds to be seen, I was forced 

 to look for something to imitate a white cloud, so that I 

 might be able to experiment at any convenient time. 

 Simple as such an invention may appear to an inventive 

 mind, it was some time before it occurred to me that a 

 white sheet of Bristol board might at times be made to 

 supply the want of a white cloud if there were otherwise a 

 sufficient quantity of atmospheric light. In sunshine or 



