46 



A POPULAR ACCOUNT 



an eye placed before the first no colour 

 is apparent. In the other case the re- 

 verse will happen, the orange light being 

 perceptible to an eye placed before, the 

 first plate, while no colour appears to an 

 eye placed behind the second. 



All that has been here observed of the 

 orange light will be equally applicable 

 to the red, yellow, and all the other co- 

 lours, the interval of the fits only being 

 different. It should, however, be ob- 

 served, that the ray does not pass sud- 

 denly into its fits of reflection and trans- 

 mission on arriving at the several points 

 of division which we have mentioned, 

 but passes gradually from its complete 

 fit of easy reflection to its complete fit 

 of easy transmission, and vice versa, 

 being, in the intermediate space, in a 

 state to be partially reflected and trans- 

 mitted. 



(60.) When'a beam of white solar light 

 falls perpendicularly on the plates, each 

 component part is put into fits separately, 

 and in the same manner as would hap- 

 pen if that part alone had been incident 

 on the plate. The thickness of the plate 

 of air may be such, that several compo- 

 nent rays may meet the second surface 

 in fits of easy reflection, while the other 



Fig 



parts meet it in fits of easy transmission. 

 In this case, the tint exhibited to an eye 

 placed before the first plate will be one 

 which is compounded of the colours of 

 those rays which meet the second plate, 

 in fits of easy reflection, while the tint 

 exhibited to an eye behind the second 

 plate will be compounded of the colours 

 of those rays which meet the second 

 surface in fits of easy transmission. It 

 will happen frequently that the second 

 surface will encounter a ray in such a 

 manner as to divide the interval of the 

 fits unequally, so that the light will be 

 partly reflected and partly transmitted. 

 In this case, the tint seen on each side of 

 the plates is determined as before, by 

 the composition of the colours reflected 

 and transmitted, due regard being had 

 to their quantities. 



(61.) Newton has given an ingenious 

 scale for determining the colours reflected 

 by the second surface, at any proposed 

 distance from the first. We shall here, 

 however, adopt another method of illus- 

 tration, not differing in principle from 

 that of Newton, but better adapted for 

 popular illustration. 



Draw two lines* AX (fig. 46) and 

 AY at right angles, and taking any 



,46. 



/ 



t S 



l n _/ 



o' 



,A 



4 B 



7 8 C 



thven points R, O, Y, G, B, I, V on the 

 efie A Y, draw through them lines pa- 

 'Mlelto AX; Let RR' represent the 

 aiterval of the fits of red light, and let 

 this space be repeated, so that R'R", 

 ri/'R"', &c. shall be equal to R R'. 

 Draw a waving curve line touching the 

 parallel through R in the points R, R', 

 il", &c., and let the points, at which the 

 iistance of the curve from the parallel 

 :s greatest, be situated exactly in the 



TO D P 3? 



middle between every two successive 



Eoints of contact, that is, perpendicu- 

 irly above the points r, r', r", &c. 

 When the thickness of the air between 

 the plates is equal to RR', or R R", or 

 R R"', &c. no red light is reflected, the 

 ray being at those points in a fit of easy 

 transmission. On Ihe other hand, in the 

 middle of each interval, or at the thick- 



* This scale is used by Mr. Herschel in his able, 

 " Treatise on Light." 



