THE WAVE THEORY OP LIGHT *R 



agation. Here is another, perpendicular to the diagram, 

 still following the law of transverse vibration; here is an- 

 other, circular vibration. Imagine a long rope, you whirl 

 one end of it and you see a screw-like motion running along, 

 and you can get this circular motion in one direction or in 

 the opposite. 



Plane-polarised light is light with the vibrations all in a 

 single plane, perpendicular to the plane through the ray 

 which is technically called the "plane of polarisation." 

 Circularly polarised light consists of undulations of lumi- 

 niferous ether having a circular motion. Elliptically polar- 

 ised light is something between the two, not in a straight line, 

 and not in a circular line; the course of vibration is an 

 ellipse. Polarised light is light that performs its motions 

 continually in one mode or direction. If in a straight line it 

 is plane-polarised; if in a circular direction it is circularly 

 polarised light; when elliptical it is elliptically polarised 

 light. 



With Iceland spar, one unpolarised ray of light divides on 

 entering it into two rays of polarised light, by reason of its 

 power of double refraction, and the vibrations are perpen- 

 dicular to one another in the two emerging rays. Light 

 is always polarised when it is reflected from a plate of un- 

 silvered glass, or from water, at a certain definite angle of 

 fifty-six degrees for glass, fifty-two degrees for water, 

 the angle being reckoned in each case from a perpendicular 

 to the surface. The angle for water is the angle whose tan- 

 gent 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 perpen- 

 dicularly to the plane of incidence and plane of reflection. 



We can distinguish it without the aid of an instrument. 

 There is a phenomenon well known in physical optics as 

 f< 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. 



