POLARIZATION OF LIGHT. 



605 



oident ray A R, as described in the experiment with 

 the apparatus, fig. 2. 



Fig. 3. 



Mere we will denominate A the polarizing plate, 

 because it is placed at the proper angle for polariz- 

 ing the light, C we will call the analyzing plate, 

 because it refuses to reflect the light which is polariz- 

 ed, but reflects that which is not polarized. For all 

 ordinary purposes the light of the sky will answer, 

 and the glasses may be adjusted by examining the 

 plate C, from the point E, and observing when a dark 

 spot in the image of the part of the sky reflected by 

 A becomes most distinctly dark, and if a large 

 quantity of polarized light be required, a bundle of 

 plates, as before described, ought to be employed, 

 instead of the single plate A. These adjustments 

 being made, take a thin plate of mica one-thirtieth 

 of an inch in thickness, or thereby, or what 

 answers still better, a similar plate of the sulphate 

 of lime, and place it between the polarizing and 

 analyzing plates, as shown at F in the figure. It 

 will now be found that the whole surface of the 

 plate A is covered with a series of most beautiful 

 colours, which vary with the inclination of the plate 

 F, or with the thickness of the portion through 

 which the ray passes. The plate F being made equal- 

 ly thick, it will be found that there are two lines D E, 

 F G, at right angles to each other, either of which 

 being in the plane of polarization R A C, or A C E, 

 the colours will disappear, and the black spot be 

 seen as if F had not been interposed between A and 

 C. These lines are called the neutral axes of the 

 crystalline plate F. On turning the plate round in 

 its own plane, the colour will increase from the 

 neutral axes till 45, when it will again diminish ; 

 the lines a b, c d are therefore the lines of maximum 

 colour, or are the depolarizing axes. If the plate F 

 be then fixed, and the plate C turned round, we will 

 find, instead of one colour, i. e. red, that we have now 

 two, red and green alternately, and each increasing 

 and then decreasing as we go nearer to or recede 

 from the depolarizing axes. It is remarkable that 

 these colours are complementary of each other, so 

 that when they are made to cross each other, white 

 is the result. It is likewise worthy of remark that 

 the kind of colour will depend on the thickness of 

 the plate of sulphate of lime, as those produced by 

 thin plates of glass, and that films, varying in thick- 

 ness from 0-00124 to 0-01818, will give all the 

 colours in Newton's table. See Newton's Optics. 



Sir D. Brewster made an experiment illustrative 

 of this subject, which we shall describe Take a 

 plate of sulphate of lime one twenty-fifth of an inch 

 in thickness, and grind one of the faces, so that it 

 shall be a triangular prism, Fi f- * 



having one of the edges as 

 thin as possible. Place this 

 prism tor a little in water, 

 it will be acted on by the 

 fluid, and a polish given to 

 its faces. This prism being 

 polarised between the plates 

 A and C, fig. 3, will, from its 

 differences of tliicknes?, 



show all the colours in Newton s table in order ; 

 when this film is cut in two in the middle, and the 

 two pieces crossed and placed between A and C 

 fringes of coloured light are likewise produced ; but 

 a new set of fringes becomes also visible at an angle 

 of 45^ to the fringes parallel to the edges of the 

 two pieces, as shown at p n, fig. 4, where a b is 

 one of the films and c d the other. 



Many singular phenomena are observable in ex- 

 amining polarized light through doubly refracting 

 crystals, both with one and two axes of double re 

 fraction. Thus take a rhomboid of calcarious spar, 

 and place upon two of its opposite faces prisms ot 

 glass, whose refracting angles are about 45, so that 

 you may be enabled to see along the axis of double 

 refraction of the spar. If this be substituted in- 

 stead of the sulphate of lime plate in the apparatus, 

 fig. 3, and let the polarized ray A C pass through 

 any part parallel to the axis of double refraction, a 

 series of most beautifully coloured concentric rings 

 will be seen, having a black cross intersecting their 

 diameters. No change will take place in these 

 rings on turning the spar round its axis, but on 

 turning the plate C changes will take place, and 

 another system of rings comes alternately into view. 



By using crystals having two axes of double re- 

 fraction, Sir D. Brewster found that two systems of 

 rings were produced. He employed nitre, which 

 crystallizes in hexagonal prisms, having angles of 

 about 120, prepared as follows: He detached with 

 a knife a small piece of the edge of the prism, and 

 ground it down till it was reduced to the eighth of an 

 inch, then smoothed its parallel faces, making them 

 perpendicular to the axis of the prism ; then having 

 wetted the faces \\ ith the tongue, he dried them 

 quickly, and placed the plate between two plates of 

 glass, and cemented them by a thin layer of Canada 

 balsam. The nitre plate, thus prepared, is put into 

 the apparatus, fig. 3, instead of F, and so situated, 

 that the plane of its axis is either perpendicular or 

 parallel to the polarizing plane RAG; then by look- 

 ing from E into the plate C,the beautiful system of 

 rings in fig. 5 will become visible. There is a black 



Fig. 5. 



cross seen in the figure, one of whose arms passes 

 through the centres of the two rings, and the other, 

 which is more faint, crosses at right angles between 

 the two systems. If the crystal be turned round, 

 the arms of the cross change, and at last open, and 

 when the turn is made 45 from the commence- 

 ment, one of the arms points with both ends up- 

 wards, and the other with both ends downwards, 

 each arm being now a hyperbolic curve, whose 



