MODERN THEORIES OF HEAT AND LIGHT 



denser and destroying the reflection, while they them- 

 selves will be more strongly propelled than if they had 

 been at rest, and the transmitted light will be increased. 

 So that the colors by reflection will be destroyed, and 

 those by transmission rendered more vivid, when the 

 double thickness or intervals of retardation are any 

 multiples of the whole breadth of the undulations ; and 

 at intermediate thicknesses the effects will be reversed 

 according to the Newtonian observation. 



" If the same proportions be found to hold good with 

 respect to thin plates of a denser medium, which is, 

 indeed, not improbable, it will be necessary to adopt 

 the connected demonstrations of Prop. IV., but, at any 

 rate, if a thin plate be interposed between a rarer and 

 a denser medium, the colors by reflection and trans- 

 mission may be expected to change places. 



Of the Colors of Thick Plates 



" When a beam of light passes through a refracting 

 surface, especially if imperfectly polished, a portion of 

 it is irregularly scattered, and makes the surface visi- 

 ble in all directions, but most conspicuously in direc- 

 tions not far distant from that of the light itself ; and if 

 a reflecting surface be placed parallel to the refracting 

 surface, this scattered light, as well as the principal 

 beam, will be reflected, and there will be also a new 

 dissipation of light, at the return of the beam through 

 the refracting surface. These two portions of scat- 

 tered light will coincide in direction ; and if the surfaces 

 be of such a form as to collect the similar effects, will 

 exhibit rings of colors. The interval of retardation is 

 here the difference between the paths of the principal 



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