SIX LECTURES ON LIGHT. 



demonstrated in tneiast lecture, of restoring 

 tne light. 



Let us now mount our Nicol prisms and 

 cross them as we crossed the tourmalines. 

 Introducing our film of gypsum between them 

 you notice tnat in one particular position the 

 film has no power whatever over the field of 

 view. But, when the film is turned a little 

 way round, the light passes. We have now 

 to understand the mechanism by which this 

 is effected. 



Firstly, then, we have this first prism which 

 receives the light emergent from the electric 

 (amp, and which is called the polarizer. Then 

 we have the p ate of gypsum, placed at S 

 (Fig. 17), and then the prism in front, which 

 is called the analyzer. On its emergence 

 from the first prism, the light is polarized ; 

 and in the particular case now before us, its 

 vibrations are executed in an horizontal plane. 

 The two directions of vibration of ; he gypsum, 

 placed at S, are now oblique to the horizon. 

 Draw a rectangular cross upon paper to rep- 

 resent the two directions of vibration within 

 the gypsum. Draw an oblique line to repre- 

 sent the intensity of the vibration when it 

 reaches the gvpsum. Let fall from the two 

 ends of this line two perpendiculars on each 

 of the arms of the cross ; then the distances 

 between the feet of these perpendiculars rep- 

 resent the intensities of two rectangular vi- 

 brations which are the equivalents of the first 

 single vibration. Thus the polarized ray, 

 when it enters the gypsum, is resolved into 

 two others, vibrating at right angles to each 

 other. 



Now, in one of those directions of vibnition 

 the ether is more sluggish than in the other ; 

 and, as a consequence, the waves that follow 

 this direction are more retarded than the 

 others. The waves of both systems, in fact, 

 are shortened when they enter the gypsum, 

 but the one system is more shortened than the 



other. You can readily imagine that in this 

 way the one system of waves may get half a 

 wave-length, or indeed any number of halt 

 wave-lengths, in advance of the other. The 

 possibility of interference here flashes upon 

 the mind. A little consideration, however, 

 renders it evident that, as long as the vibra- 

 tions are executed at right angles to each 

 other, they cannot quench each other, no 

 matter what the retardation may be. This 

 brings us at once to the part played by the 

 analyzer. Its sole function is to recompound 

 i he two vibrations emergent from the gypsum. 

 It reduces them to a single plane, where, if 

 one of them be retarded by the proper 

 amount, extinction can occur. But here, as 

 in the case of thin films, the different lengths 

 of the waves of light come into play. Red 

 will require a greater thickness to produce the 

 retardation necessary for extinction than blue; 

 consequently, when the longer waves have 

 been withdrawn by interference, the shorter 

 ones remain and confer their colors on the 

 film of gypsum. Conversely, when the 

 shorter waves have been withdrawn, the 

 thickness is such that the longer waves re- 

 main. An elementary consideration suffices 

 to show that, when the directions of vibration 

 of prisms and gypsum enclose an angle of 

 forty-five degrees, the colors are at their 

 maximum brilliancy. When the film is 

 turned from this direction, the colors gradu- 

 ally fade, until, at the point where the direc- 

 tions are parallel, they disappear altogether. 



A knowledge of these phenomena ':*> best 

 obtained by means of a model of wood or 

 aasteboard representing the plate of gypsum, 

 ts planes of vibration, and also those of the 

 Dolarizer and analyzer. On these planes the 

 waves may be drawn, showing the resolution 

 of the first polarized ray into two others, and 

 hen the reduction of tne two vibrations to a 

 common plane. Following out ligidly the 

 nte/action of the two systems of waves, we 

 are taught by such a model that a'l the phe- 

 nomena of color, obtained, when the planes of 

 vibration of the two Nicols are parallel, are 

 displaced by the complementary phenomena 

 when the Nicols are perpendicular to each 

 other. 



In considering the next point, for the sake 

 of simplicity, we will operate with monochro- 

 matic light with red light, for example. 

 Supposing that a certain thickness of the gyp- 

 sum produces a retardation of half a wave- 

 length, twice this thickness will produce a 

 retaidation of two half wave-lengths; three 

 times this thickness a retardation of three 

 half wave-lengths, and so on. Now, when 

 the Nicols are parallel, the retardation of 

 half a wave-length, cr of any odd number D 

 half wave-lengths, produces extinction; at ail 

 thicknesses, on the other hand, which corre- 

 spond to a retardation of an even number of 

 half wave-lengths, the two beams support each 

 other, when they are brought to a common 

 plane by the analyzer. S-pposing, then, 



