AND THE THEORY OF LIGHT. 79 



class of experiments. It is the shadows which these discs 

 cast on the white wall of a room, or on a white sheet of 

 paper, which were investigated. 



On taking one of the discs of the second class of expe- 

 riments, that is discs with open spaces to transmit light, 

 and placing it so that it should cast its shadow, either in 

 sunshine or in gaslight, on a sheet of white paper, and 

 then making it revolve, the shadows were seen vividly 

 illuminated. The colours in this class of experiments were 

 more distinct than those of the other two classes. They 

 exhibited, however, precisely the same phenomena under 

 a different phase, but they had this advantage, that they 

 could not easily escape the notice of the most careless 

 observer, for they had the appearance of being painted 

 on paper, in the same way as the solar spectrum appears 

 painted ; hence I call them solar spectra by reflection. 



121. If the experiments are made under a jet of gas, the 

 shadows will have very marked penumbrse before being 

 put in motion, and if when in motion the penumbrse, 

 which are now coloured, are carefully examined, it will be 

 seen how closely such phenomena are allied to the colours 

 produced by prismatic refraction. In fact they demon- 

 strate that the colours of the prism are coloured penum- 

 brse. They give a complete proof of the identity of the 

 two phenomena. 



122. Newton attempts to prove that the penumbrse have 

 nothing to do with the colours of the solar spectrum. He 

 is constantly reverting to the proposition which he is 

 straining his faculties to demonstrate, " That colours are 

 to be derived from some other cause than the new modifi- 

 cations of light by refractions and shadows.^' He acknow- 

 ledges that there is a " penumbra," to use his own words, 

 " made in the circuit of the spectrum Y," that is before 

 refraction, " and that penumbra remains in the rectilinear 

 sides of the spectrums P T and p t," that is after refrac- 



