30 



SIX LECTURES ON LIGHT. 



that we take a plate of a wedge-form, which 

 grows gradually thicker from edge to back, 

 we ought to expect in red light a series of 

 recurrent bands of light and darkness ; the 

 dark bands occurring at thicknesses which 

 produce retardations of one, three, five, etc., 

 half wave-lengths, uhile the light bands occur 

 between the dark ones. Expeiiment proves 

 the wedge-shaped crystal to show these bands 

 but they are far better shown by this circular 

 film, which is so worked as to be thinnest at 

 the centre, gradually increasing in thickness 

 from the centre outwards. These splendid 

 rings of light and darkness are thus produced. 

 When, instead of employing red light, we 

 employ blue, the rings are also seen ; but as 

 they occur at thinner portions of the film, 

 they are smaller than the rings obtained with 

 the red light. The consequence of employ- 

 ing %uhite light may now be inferred : inas- 

 much as the red and the blue fall in different 

 places, we have iris-colored rings produced by 

 the white light. 



Some of the cVomatic effects of irregular 

 crystallization are beautiful in the extreme. 

 Could I introduce between our Nicols a pane 

 of glass covered by those frost-ferns which 

 the cold weather renders now so frequent, 

 rich colors would be the result. The beauti- 

 ful effects of irregular crystallization on glass 

 plates, row presented to you, illustrate what 

 you might e.\pect from the irosted window- 

 pane. And not only do crystalline bodies 

 act thus upon light, but almost all bodies that 

 possess a definite structure do the same. As 

 a general rule, organic bodies act in this way; 

 for their architecture implies an arrangement 

 of the ether which involves double refraction. 

 A film of horn, or the section of a shell, for 

 example, yields very beautiful colors in polar- 

 ized light. In a tree, the ether certainly pos- 

 sesses different degrees of elasticity along and 

 across the fibre; and, were wood transparent, 

 this peculiarity of molecular structure would 

 infallibly reveal itself by chromatic phe- 

 nomena like those that you have seen. But 

 not only do todies built permanently by 

 Nature behave in this way, but it is possible, 

 as shown by Brewster, to confer, by strain or 

 by pressure, a temporary double-refracting 

 structure upon non-crystalline bodies, such as 

 common glass. 



When I place this bar of wood across my 

 knee and seek to break it, what is the 

 mechanical condition of the bar ? It bends, 

 and its convex surface is strained longitudi- 

 nally; its concave surface, that next my knee, 

 is longitudinally pressed. Both in the 

 strained portion and in the pressed portion 

 the ether is thrown into a condition which 

 would rend r the wood, were it transparent, 

 double refracting. Let us repeat the experi- 

 ment with a bar of glass. Between the 

 crossed Nicols I introduce such a bar. By 

 the dim residue of light lingering upon the 

 screen, you see the image of the glass, but it 

 has no effect upon the light. I simply bend 



the gla^s bar with my finger and thumb, 

 keeping its length oblique to the directions of 

 vibration in the Nicols. Instantly light 

 flashes out upon the screen. The two sides 

 of the bar are illuminated, the ed.jes most, 

 for here the strain and pressure are greatest. 

 In passing from strain to pressure, we cross a 

 portion of the glass where neither is exerted. 

 This is the so-called neutral axis of the bar 

 of glass, and along it you see a dark band, 

 indicating that the j;lass along this axis exer- 

 cises no . ction upon the light. By employ- 

 ing the force of a press, instead of the lorce 

 of my finger and thumb, the brilliancy of the 

 light is greatly augmented. 



Again, I have here a square of glass which 

 can be inserted into a press of another kind. 

 Introducing the square between the prisms, 

 its neutrality is declared ; but it can hardly 

 be held sufficiently loosely to prevent its 

 action from manifesting itself. Already, 

 though the pressure is infinitesimal, you see 

 spots of light at the points where the press is 

 in contact with the glass. I now turn this 

 screw. Instantly the image of the square of 

 glass flashes out upon the screen. You see 

 luminous spaces separated from each other 

 by dark bands. Every pair of adjacent 

 luminous spaces is in opposite mechanical 

 conditions. On one side of the dark band 

 we have strain, on the other side pressure ; 

 while the dark band marks the neutral axis 

 between both. I now tighten the vise, and 

 you see color ; tighten still more, and the 

 colors appear as rich as those presented by 

 crystals. Releasing the vise, the colors 

 suddenly vanish ; tightening suddenly, they 

 reappear. From the colors of a soap-bubble 

 Newton was able to infer the thickness of the 

 bubble, thus uniting by the bond of thought 

 apparently incongruous things. From the 

 colors here presented to you, the magnitude 

 of the pressure employed might be inferred. 

 Indeed, the late M. Wertheim, of Paris, in- 

 vented an instrument for the determination 

 of strains and pressures by the colors of 

 polarized light, which exceeded in accuracy 

 all other instruments of the kind. 



You know that bodies are expanded by 

 heat and contracted by cold. If the heat be 

 applied with perfect uniformity, no local 

 strains cr pressures come into play ; but, if 

 one portion of a solid be heated and others 

 not, the expansion of the heated portion intro- 

 duces strains and pressures which reveal 

 themselves under the scrutiny of polarized 

 ight. When a square of common window- 

 jlass is placed between the Nicols, you see 

 ts dim outline, but it exerts no action on the 

 polarized light. Held for a moment over the 

 :lame of a spirit-lamp, on reintroducing it 

 between the Nicols, light flashes out upon 

 the screen. Here, as in the case of mechan- 

 cal action, you have spaces of strain divided 

 3y neutral axes from spaces of pressure. 



Let us apply the heat more symmetrically. 

 This small square of glass is perforated at 



