118 COLORS OF THIN PLATES. 



themselves wlicn two doubly refracting rhombs are combined — appearances 

 which were observed by him v/ith surprise and perplexity. They are now 

 known to be owing to a remarkable moditication of light which always accom- 

 panies double refraction, though it may be produced in other ways, and which 

 is ctAlcdi polarization. This v,'i]l occupy much of our attention further- on. 



Soon after his announcement of the compound nature of light, Sir Isaac 

 Newton made public the results of his ingenious investigations in regard to the 

 colors esrhibited by thin plates of transparent substances, such as soap-bubbles, 

 films of moisture upon glass and upon polished opaque solids, lamiuai of air 

 confined in fissures of transparent minerals, S.CQ,. lie showed that the tints dis- 

 played by such thin plates, when viewed in common light, depend upon three 

 conditions, viz : the riiickncss of the plate, its refracting power, and the angle 

 of obliquity under which it is viewed. The determination of the relation of the 

 tint to the thickness, was made by means of a very simple contrivance. A 

 double-convex lens, of very long focus, was placed in contact with the plane 

 surface of a plano-convex lens, the two being pressed together by means of 

 screws. In Newton's experiments the double-convex lens was beneath and the 

 plano-convex above. The convexity of the upper surface of the upper lens is 

 advantageous when oblique observations are desired, as tending to reduce the 

 refraction of the incident and emergent rays at that surface. The two touching 

 sui-faces have, theoretically, but a single point of contact, and that point is the 

 centre of a thin plate of included air, of which the thick- 

 ness increases from zero equally in all directions. The 

 law of this increase will be apparent from the figure 

 annexed. MN represents the lower surface of the supe- 

 rior glass, and QR the upper surface of the inferior. Let 

 C be the centre of the sphere of vrhich QR is a super- 

 ficial section. Put r for the radius CP. Then, if the 

 arcs Pa, Pb, are small in proportion to the whole circum- 

 ference, we shall have 



Pa'=Aa^'^; imdi Ph'=Bb=.^^ ■ 



Or, if X stand generally for the thickness Aa or Bb, and y for the corres- 

 ponding distance from the point of contact, PA or PB, we shall have the vari- 

 ation, xccy^. 



This furnishes a law by which, when the thickness corresponding to a single 

 assigned value of y is known, the thickness for all other values may be com- 

 puted with great facility. 



The apparatus being arranged as above described, the colors which are seen 

 by reflected light are arranged in regular rings around a black centre and iu suc- 

 cessive series, as follows : 



1. Black, blue, white, yellow, red. 



2. Violet, blue, green, yellow, red. 



3. Purple, blue, green, yellow, red. 



4. Green, red. 



5. Greenish blue, red. 



6. Greenish blue, pale red, 



7. Greenish blue, reddish white. 



These are what Newton calls the successive orders of colors, and, in referring 

 lo any particular tint, it is designated as the blue, i-ed, green, &c., of the first, 

 second, or third order, as the case may be. Beyond the fourth order the colors 

 become feeble or begin to fade rapidly out into whiteness, and, beyond the 

 seventh, color can scarcely be at all perceived. The cause of this fading may 

 be made manifest by employing hotnogencous or monochromatic light; that is 

 to say, light of a single tint only, obtained by isolating a portion of the rays of 



