148 COLOURS OF THIN PLATES. 



of Newton's scale. Their diameters increase with, the distance 

 of the plate from the focus, and diminish as the thickness of 

 the plate increases. They are formed, as before, by the in- 

 terference of the two pencils, which are scattered in passing 

 through the two surfaces of the plate. 



The phenomenon is reduced to its simplest conditions, 

 by receiving the light diverging from a narrow rectilinear 

 aperture upon two polished wires, stretched parallel to the 

 slit, and nearly in the same plane with it. If then the eye, 

 fortified by a lens, be placed so that the sum of the dis- 

 tances of the aperture and of the eye from each wire is the 

 same, a series of coloured fringes will be seen, formed by 

 the interference of the pencils irregularly reflected by the 

 two wires. 



(162) When the interval between two glasses is filled 

 with different substances, such as water and air, or water 

 and oil, in a finely subdivided state, the portions of light 

 which have traversed them are in a condition to interfere, 

 the interval of retardation depending on the difference of the 

 velocities of light in the two media. Accordingly, coloured 

 rings will be seen, when a luminous object is viewed through 

 the glasses, the rings being similar to those usually seen by 

 transmission, but much larger. But when a dark object is 

 behind the glasses, and the incident light somewhat oblique, 

 the rings immediately change their character, and resemble 

 those of the ordinary reflected system ; one of the portions, 

 in this case, being reflected, and therefore suffering a loss of 

 half an undulation. 



These phenomena were observed and explained by Young, 

 and were denominated by him the colours of mixed plates. 

 Young observed some similar effects in an unconfined medium. 

 Thus, when the dust of the lycoperdon is mixed with water, 

 the mixture exhibits a green tint by direct light, and a purple 



