334 POPULAR LECTURES AND ADDRESSES. 



perpendicular to the surface. The angle for water 

 is the angle whose tangent is 1*4. I wish you to 

 look at the polarisation with your own eyes. Light 

 from glass at fifty-six degrees and from water at 

 fifty-two degrees goes away vibrating perpendi- 

 cularly to the plane of incidence and plane of 

 reflection. 



We can distinguish it without the aid of an in- 

 strument. There is a phenomenon well known 

 in physical optics as " Haidinger's Brushes." The 

 discoverer is well known in Philadelphia as a 

 mineralogist, and the phenomenon I speak of goes 

 by his name. Look at the sky in a direction of 

 ninety degrees from the sun, and you will see a 

 yellow and blue cross, with the yellow toward the 

 sun, and from the sun, spreading out like two 

 foxes' tails with blue between, and then two red 

 brushes in the space at right angles to the blue. 

 If you do not see it, it is because your eyes are 

 not sensitive enough, but a little training will give 

 them the needed sensitiveness. If you cannot see 

 it in this way try another method. Look into a 

 pail of water with a black bottom ; or take a clear 



