20 



POLARISATION OF LIGHT. 



may be calculated from the last column 

 of Newton's table, under Glass, because 

 glass and sulphate of lime have nearly 

 the same refractive power. Since 

 the white in the column of reflected 

 tints is produced at a thickness of 

 0.00124, and the white or end of the 

 seventh spectrum is produced at a thick- 

 ness of 0.01818, and since the numbers 

 in the table opposite to these tints are 3 

 and 49f, we have the following propor- 

 tion : as 0.018180.00124 or .01694 is to 

 49 or 3, or 46 nearly, so is the excess 

 of any thickness of a plate of sulphate of 

 lime above 0.0124 to a fourth number, 

 which when added to 3f , will give a sum 

 to enter the last column, and opposite to 

 it will be found the two complemen- 

 tary colours which such a plate will pro- 

 duce. 



As the colours of polarised light are 

 proportional to the thickness of thin 

 plates, which give the same colours, the 

 superposition "of two films will have the 

 same effect as one film, equal to their 

 united thicknesses, provided they are 

 laid together in the same manner as they 

 lie in the crystal. But if the films are 

 placed transversely, that is with any one 

 line of the one at right angles, to a similar 

 line in the other, they will produce a tint 

 equal to the difference of their thick- 

 nesses. If the plates .are therefore perfectly 

 equal, they will, when thus crossed, destroy 

 each other's action, and produce black- 

 ness. Hence, also, the colours may be 

 produced by crossing two plates of very 

 considerable thickness, which give no 

 colour when taken separately, provided 

 that the difference of their thickness 

 does not exceed 0.018,18 of an inch *. 



The different phenomena which we 

 have now described, may be seen in a 

 more instructive manner, in the follow- 

 ing experiment made by Dr. Brewster. 

 Having taken a plate of sulphate of lime 

 of equal thickness, and about ^th of an 

 inch thick, he ground down one of its 

 faces, so as to make its thickness vary 

 from 5^th of an inch, down to the thin- 

 nest edge that could be made. He then 

 placed the plate in water, which slowly 

 acted upon it, making its edge thinner, 

 and giving a slight polish to its surface. 

 By placing this film between the polaris- 

 ing and analysing plate, its surface was 

 covered with coloured fringes parallel to 

 the thin edge, a d,Jig. 22, and including 



* The preceding interesting results were first ob- 

 tained l.y M. hmt. 



Fig. 22. 



all the colours in Newton's Table, thus 

 showing to the eye how the different 

 tints are produced by different thick- 

 nesses. When the film a d, was cut into 

 two a b, c d, and crossed as in Jig. 23, a 

 Fig. 23. 



new set of fringes was produced parallel 

 to a black line n p, extending from the 

 point where the two thinnest edges meet 

 to the point where the two thickest sides 

 meet. 



On the Colours and Systems of Rings 

 produced by Crystals, with one Axis of 

 Double Refraction. The phenomena of 

 the colours of polarised light had been 

 examined under very unfavourable cir- 

 cumstances, till 1813, when the systems 

 of rings round the axis of double refrac- 

 tion, were discovered by Dr. Brewster. 



If we take a rhomboid of calcareous 

 spar, whose principal section is repre- 

 sented by AB CD, fig. 24, and cement 

 upon its surfaces A B, C D, two 

 Fig. 24. 



prisms BEF, DEH, having their refract- 

 ing angles EBF, GDH about 45, we 

 shall be able to see along the axis B b, 

 of double refraction of the spar. Let the 

 spar be now substituted in the apparatus 

 fig. 21, in place of the film D FE G, so 

 that the polarised ray A C,Jig. 21, may 

 pass along a line parallel to the axis B b ; 

 then whatever be the position of the spar, 

 there will be seen along its axis A B, a 

 most beautiful system of coloured rings, 

 intersected in the direction of their dia- 

 meters by a black cross A B C D, Jig. 25. 



