SIX LECTURES ON LIGHT. 



the centre, and into the orifice a bit of copper 

 wire is intioduced. Placing the square be- 

 tween the prisms, and heating the copper, 

 thft heat passes by conduction along the wire 

 to the glass, through which it spreads from 

 the centre outwards. You sse a dim cross 

 bounding four luminous quadrants growing 

 up and becoming gradually black by compari- 

 son with the adjacent brightness. And as, in 

 the case of pressure, we produced colors, so 

 here also, by the proper application of heat, 

 gorgeous chromatic effects may be produced, 

 and they may be rendered permanent by first 

 heating the glass sufficiently, and then cool- 

 ing it, so that the chilled mass shall remain 

 in a state of strain and pressure. Two or 

 three examples will illustrate this point. The 

 colors, you observe, are quite as rich as those 

 obtained in the case of crystals. 



And now we have to push these considera- 

 tions to a final illustration. Polarized light 

 may be turned to account in various ways as 

 an analyzer of molecular condition. A strip 

 of glass six feet long, two inches wide, and 

 a quarter of an inch thick, is held at the 

 centre between my finger and thumb. I 

 sweep over one of its halves a wet woolen 

 rag ; you hear an acute sound, due to the 

 vibrations of the glass. What is the condi- 

 tion of the glass while the sound is heard ? 

 This . its two halves lengthen and shorten in 

 quick succession. Its t\vo ends, therefore, 

 aie in a state of quick vibration ; but at the 

 centre the pulses from the two ends alter- 

 nately meet and retreat. Between their 

 opposing actions, the glass at the centre is 

 kept motionless ; but, on the other hand, it 

 is alternately strained and compressed. The 

 state of the glass may be illustrated by a row 

 of spots of light, as the propagation of a 

 sonorous pulse was illustrated in a former 



FIG. 1 8. 



lecture. By a simple mechanical contrivance 

 the spots are made to vibrate to and fro. 

 The terminal dots have the largest amplitude 



of vibration, while those at the centre are 

 alternately crowded together and torn 

 asunder, the centre one not moving at all. 

 The condition of the sounding strip of glass 

 is here correctly represented. In Fig. 18, A 

 B represents the glass rectangle with its 

 centre condensed ; while A' B' represents the 

 same rectangle with its centra rarefied. 



If we introduce the glass s s' (Fig. 19) be- 

 tween the crossed Nicols, taking care to keep 

 the strip oblique to the direction of vibration 

 of the Nicols, and sweep our wet rubber over 

 the glass, this may bi expected to occur : At 

 every moment of compression the light will 

 flash through ; at every moment of strain the 

 light will also ilash through ; and these states 

 of strain and pressure will follow each other 

 so rapidiy that we may expect a permanent 

 luminous impression to be made upon the 

 eye. By pure reasoning, therefore, we reach 

 the conclusion that the light will be revived 

 whenever the glass is sounded. That it is so, 

 experiment testifies : at every sweep of the 

 rubber, a fine luminous disk (o) flashes out 

 upon the screen. The experiment may be 

 varied in this way : Placing in fron of the 

 polarizer a plate of unannealed glass, you 

 have those beautiful colored rings, intersected 

 by a black cross. Every sweep of the rubber 

 not only abolishes the rings, but introduces 

 complementary ones, the black cross bein^ 

 for the moment supplanted by a white one. 

 This is a modification of an experiment 

 which we owe to Biot. His apparatus, how- 

 ever, confined the observation of it to a single 

 person at a time. 



But we have to follow the ether still 

 further. Suspended before you is a pendu- 

 lum, which, when drawn aside and then 

 liberated, oscillates to and fro. If when the 

 pendulum is passing the middle point of its 

 excursion, I impart a shock to it tending to 

 drive it at right angles to its present course, 

 what occurs? The two impulses compound 

 themselves to a vibration oblique in direction 

 to the former one, but the pendulum oscil- 

 lates in a plane. But, if the rectangular 

 -hock be imparted to the pendulum whpn it 

 is at the limit of its swing, then the com- 

 pounding of the two impulses causes the sus- 

 pended ball to describe not a straight line, 

 but an ellipse ; and, if the shock be compe- 

 tent of itself to produce a vibration of the 

 same amplitude as the first one, the ellipse 

 becomes a circle. But why do I dwell upon 

 these things ? Simply to make known to you 

 the resemblance of these gross mechanical 

 vibrations to the vibrations of light. 1 hold 

 in my hand a plate of quartz cut from the 

 crystal perpendicular to its axis. This crys- 

 tal thus cut possesses the extraordinary power 

 of twisting the plane of vibration of a. polar- 

 ized ray to an extent dependent on the thick- 

 ness of the crystal. And the more refrangi- 

 ble the light the greater is the amount of 

 twisting, so that, when white light is em- 

 ployed, its constituent colors are thus drawo 



