3G .OPTICS. 



may be a true one, even if light should the rings corresponding to it will disap- 

 consist of material particles, seems to pear. Hence it is manifest, that the 



aree well with the experiments of Sir 

 Isaac Newton. 



_ TTT7 . ~ . '. , mi I, 

 CHAPTER XIV. -Colours of Thick 



Plates-of Double ***<*/ Equal 



Thickness-of Double Plates of Un- Howeyer rfectl 



equal Thickness 



Colours of Single Thick &&-** 

 colours produced by glass m rrors or 



first examined by Sir Isaac New- 

 on. Having admitted a beam of the 

 sun R R (fig. 37.) through a hole in 

 Fig. 37. 



his window shutter, one-third of an inch 

 in diameter, he threw it in the direction 

 of its axis on a glass mirror M, a 

 quarter of an inch t hick,' concave in 

 front, and convex and quicksilvered be- 

 hind ; the radius of the curvature of 



back of the mirror is necessary to the 

 production of the rings ; an inference 

 which is also proved by the fact, that 

 they never appear when the reflexion 



metallic surface. 



any surface of 



glass or metal is polished, it scatters 

 irregularly in every direction a faint 

 light, by means of which the polished 

 surface, when illuminated in a dark 

 room by a beam of the sun's light, may 

 be seen in all positions of the eye. The 

 colours of thick plates are* obviously 

 produced by this scattered 'light. Sir 

 Isaac Newton explains the colours, by 

 the hypothesis of fits of easy reflexion 

 and transmission, mentioned in the pre- 

 ceding chapter ; while Dr. Young as- 

 cribes them to the interference of two 

 portions of light, one scattered by the 

 front surface of the mirror before re- 

 fraction, and the other scattered by the 

 same surface when the ray returns to 

 it after reflexion from the back surface. 

 Colours of Double Plates of Equal 

 Thickness. In the preceding experi- 

 ments the rings are produced by a 



both sides being 5 feet 11 inches. When sinj?le plate . \, ut t h ey may be more 

 a sheet of paper was held 5 feet 11 easily seen, and with much more brilli- 

 inches in front of the mirror, he disco- ancy> when they are pro duced by suc 



veredfour or Jive coloured rings round 

 the aperture R. These rings had the 

 same colours as those seen by light 

 transmitted through thin plates, and 

 described in the preceding chapter. 

 When the light R R was red, all the 

 rings were red, and so on with the other 

 colours ; the rings being largest in red, 

 and smallest in violet light. Upon 

 measuring their diameter in homoge- 

 neous light, Sir Isaac found that the 

 squares of the diameters of the most 

 luminous parts were as the numbers 0, 

 1, 2, 3, 4, 5, &c., and the squares of 

 the diameters of the darkest parts, as 

 the intermediate numbers i, H, 2i, 3i, 

 4, &c. When mirrors of greater thick- 

 nesses were used, the rings grew less 

 and less, and their diameters werereci- 



cessive reflexions from two plates of 

 equal thickness. This phenomenon, 

 which was discovered by Dr. Brewster, 

 will be understood from (fig. 380, 



Fig. 38. 



where A B, C D are sections of two 



procally as the square roots of the thick- plates of parallel glass cut out of the 

 ness of the mirror. If we rub off the same piece. Let the distance of their 

 quicksilver from the back of the mirror, nearest surfaces be about ^th of an inch; 



the rings become fainter, without al- 



tering their magnitude; and if we 



place a large drop of oil of turpentine 



on the back of the mirror deprived of 



its silvering, or any other oil of the same light subtending an angle of 2 or 3. 



refractive power r.s the glass, we shall This circular disk will be seen single ; 



remove entirely the reflective power of but, if one of the plates be gently in- 



that part of the surface, and the part of clined to the other, as shown in the 



and the eye being placed behind them 

 atV, let the observer look through them 

 in the direction V R at a candle, or, 

 what is better, at a circular disk of 



