THE WAVE THEORY OF LIGHT. 335 



glass dish of water, rest it on a black cloth, and 

 look down at the surface of the water on a day, 

 with a white cloudy sky (if there is such a thing 

 ever to be seen in Philadelphia). You will see 

 the white sky reflected in the basin of water at an 

 angle of about fifty degrees. Look at it with the 

 head tipped on one side and then again with the 

 head tipped to the other side, keeping your eyes 

 on the water, and you will see Haidinger's brushes. 

 Do not do it fast or you will make yourself giddy. 

 The explanation of this is the refreshing of the 

 sensibility of the retina. The Haidinger's brush 

 is always there, but you do not see it because your 

 eye is not sensitive enough. After once seeing it 

 you always see it ; it does not thrust itself in- 

 conveniently before you when you do not want 

 to see it. You can also readily see it in a piece 

 of glass with a dark cloth below it, or in a basin 

 of water. 



I am going to conclude by telling you how we 

 know the wave-lengths of light, and how we know 

 the frequency of the vibrations, and we shall actu- 

 ally make a measurement of the wave-length of 



