CLOUDS AND RIVERS, ICE AND GLACIERS. 9 



bodies have the power of affecting the retina — but 

 how? 



23. It was long supposed that from such bodies 

 issued, with inconceivable rapidity, an inconceivably fine 

 matter, which flew through space, passed through the 

 pores supposed to exist in the humours of the eye, 

 reached the retina behind, and by their shock against 

 the retina, aroused the sensation of light. 



24. This theory, which was supported by the greatest 

 men, among others by Sir Isaac Newton, was found 

 competent to explain a great number of the phenomena 

 of light, but it was not found competent to explain all 

 the phenomena. As the skill and knowledge of experi- 

 menters ii; creased, large classes of facts were revealed 

 which could only be explained by assuming that light 

 was produced, not by a fine matter flying through space 

 and hitting the retina, but by the shock of minute waves 

 against the retina. 



25. Dip your finger into a basin of water, and cause 

 it to quiver rapidly to and fro. From the point of dis- 

 turbance issue small ripples which are carried forward 

 by the water, and which finally strike the basin. Here, 

 in the vibrating finger, you have a source of agitation ; 

 in the water you have a vehicle through which the 

 finger's motion is transmitted, and you have finally the 

 side of the basin which receives the shock of the little 

 waves. 



26. In like manner, according to the wave theory of 



