SIX LECTURES ON LIGHT. 



one of these beams, we might employ the 

 other for experiments on polarized light. 



These beams, as you know, are refracted 

 differently, and from this we are able to infer 

 that under some circumstances the one may 

 be totally reflected, and the other not. An 

 optician, named Nicol, cut a crystal of Ice- 

 land spar in two in a certain direction. He 

 polished the severed urfaces, and reunited 

 them by Canada balsam, the surface of union 

 being- so inclined to the beam traversing the 

 spar that the ordinary ray, which is the most 

 highly refracted, was totally reflected by the 

 balsam, while the extraordinary ray was per- 

 mitted to pass on. The invention of the 

 Nicol prism was a great step in practical op- 

 tics, and quite recently such prisms have 

 been constructed of a size which enables 

 audiences like the present to witness the 

 chromatic phenomena of polarized light to a 

 degree altogether unattainable a short time 

 ago. The two prisms here before you belong 

 to my excellent friend, Mr. William Spottis- 

 woode, and they were manufactured by Mr. 

 Ladd. I have with me another pair of very 

 noble prisms, still larger than these, manu- 

 factured for me by Mr. Browning, who has 

 gained so high and well-merited a reputation 

 in the construction of spectroscopes. 



These two Nicol prisms play the same 

 part as the crystals of tourmaline. Placed 

 with their directions of vibration parallel, 

 the light passes through both. When these 

 directions are crossed, the light is quenched. 

 Introducing a film of mica between the 

 prisms, the light is in part restored. But 

 notice, when the film of mica is thin, you 

 have sometimes not only light, but colored 

 light. Our work for some time to come will 

 be the examination of these colors. With 

 this view, I will take a representative crystal, 

 one easily dealt with; the crystal gypsum, or 

 selenite, which is crystallized sulphate of 

 lime. Between the crossed Nicols I place a 

 thick plate of this crystal; like the mica, it 

 restores the light, but it produce-^ no color. 

 With my penknife I take a thin splinter from 

 this crystal and place it between the prism;- { 

 its image on the screen glows with the richest 

 colors. Turning the prism in front, these 

 colors gradually fade, disappear, but by con- 

 tinuing the rotation until the vibrating sec- 

 tions of the prisms are parallel, vivid colors 

 again appear, but these colors are comple- 

 mentary to the former ones. 



Some patches of the splinter appear of one 

 color, some of another. These differences 

 ar.: due to the different thicknesses of the. 

 film. If the thickness be uniform, the color 

 is uniform. Here, for instance, is a stellar 

 shape, every lozenge of the star being a film 

 of gypsum of uniform thickness. Each 

 lozenge, you observe, shows a brilliant uni- 

 fcrm color. It is easy, by shaping our films 

 so as to represent flowers or other objects, 

 to exhibit such objects in colors unattainable 

 by art. Here, for example, is a specimen of 



heart's-ease, the colors of which you might 

 safely defy the artist to reproduce. By turn- 

 ing the front Nicol ninety degrees round, we 

 pass through a colorless phase to a series of 

 colors complementary to the foroier ones. 

 Here, for example, is a rose tree with red 

 flowers and green leaves; turning the prism 

 ninety degrees round, we obtain a green 

 flower and red leaves. All these wonderful 

 chromatic effects have definite mechanical 

 causes in the motions of the ether. The 

 principle of interference, duly applied and 

 interpreted, explains them all. 



By this time you have learned that the 

 word " light" may be used in two different 

 senses ; it may mean the impression made 

 upon consciousness, or it may mean the phys- 

 ical agent which makes the impression. It 

 is with the agent that we have to occupy our- 

 selves at present. That agent is the motion 

 of a substance which fills ail space, and sur- 

 rounds the atoms and molecules of bodies. 

 To this interstellar and interatomic medium 

 definite mechanical properties are ascribed, 

 and we deal with it as a body possessed of 

 these p operties. In mechanics we have the 

 composition and resolution of forces, and of 

 motions, extending to the composition and 

 resolu.ion of vibrations. We treat the lumi- 

 niferous ether on mechanical principles, and 

 from the composition, resolution, and inter- 

 ference of its vibrations, we deduc; all the 

 phenomena displayed by crystals in polarized 

 light. 



Let us take, as an example, the crystal of 

 tourmaline, with which we are now so famil- 

 iar. Let a vibration cross this erysta* oblique 

 to its axis ; \vj have seen by experiment tnat 

 a portion of the light will pan through. 

 How much, we determine in this way : Draw 

 a straight line representing the intensity of 

 the vibration before it reaches the tourmaline, 

 and from the two ends of this line draw two 

 perpendiculars to the axis of the crystal ; the 

 distance between the feet of these two per- 

 pendiculars will represent the intensity 01 Lae 

 transmitted vibration. 



Follow me now while I endeavor to make 

 clear to you what occurs when a film of 

 gypsum is placed between the JNicol pri>ms. 

 But, at the outset, let us cstabhsn still 

 further the analogy between the action of the 

 prisms and that ot two plates of tourmaline, 

 The plates are now crossed, and you see that 

 by turning the film round, it may be placed 

 in a position where i' has no povver to -abolish 

 the darkness. Why is this? The answer is 

 that \<\ the gypsum there are two directions, 

 at light angles to each other, which, the waves 

 of light are constrained to follow, ;:nd that 

 now one of these directions is parallel to one 

 of the axes of the tourmaline, and the oiher 

 parallel to the other axis. When this is the 

 case, the film exercises no sensible action 

 upon the light. But now I turn the film so 

 as to render its direction of vibration oblique 

 to the axes ; then you see it has the power, 



