AND THE THEOilY OF LIGHT. 83 



is here contending with, viz. : the cause of the spectrum 

 being white when it first emerges from the prism. 



125. Let us now revert for a moment to the phenomena 

 of Class II.j caused by what I have called transmitted 

 light, and they will be seen to be analogous to those pro- 

 duced by transparent coloured media. The holes in the 

 card may be considered as representing the pores of the 

 substance, and the change of the angle of the paper re- 

 flecting the light, or of the figure transmitting the light, 

 may be considered as corresponding to the difference of 

 arrangement in the molecules of dififerent solids, which is 

 the cause of their refracting (as it is usually termed) 

 light differently. 



The experiments of Class III. are similar to those of 

 Class II. ; in one case the images are depicted on paper, 

 in the other directly on the eye. But in those of Class 

 III. the penumbras are seen on the screen before motion 

 is given to the machine. In Class II., although the pe- 

 numbrse are there, before motion is produced, they are 

 not perceived by the eye. I consider the phenomena of 

 Class III. to be similar in many respects to prismatic 

 phenomena, and when we compare those of the second 

 and third classes together, we can understand why pe- 

 numbrse are not seen in the prism, and at the same time 

 learn how essential they are to the formation of colour. 



These two classes of experiments clear away much of 

 the mysterious nature of the prism. When we come to 

 the phenomena caused by perpendicular motion, the sub- 

 ject will be made clearer still. Other difficulties will be 

 solved, but the prism would require a paper for itself. 



I have not thought it necessary to give any diagrams, 

 coloured or otherwise, to illustrate Classes II. and III. 

 of experiments. They are similar to those already given, 

 as far as colour is concerned. 



