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of mixed plates in looking at a candle through two pieces 

 of plate-glass, with a little moisture between them. I ob- 

 served an appearanee of fringes resembling the common 

 colours of thin plates j and upon looking for the fringes by 

 reflection, I found that these new fringes were ahvays in the 

 same direction as the other fringes, but many times larger. By 

 examining the glasses with a magnifier, I perceived that wher- 

 ever these fringes were visible, the moisture was iutermixed 

 with portions 'of air, producing an appearanee similar to 

 dew. I then supposed that the origin of the colours was 

 the same as that of the colours of halos ; but, on a more 

 minute examination, I found that the magnitude of the 

 portions of air and water was by no means uniform, and 

 that the explanation was therefore inadmissible. It was, 

 however, easy to find two portions of light sufficiënt for 

 the production of these fringes; for, the light transmitted 

 through the water, moving in it with a velocity different 

 from that of the light passing through the interstices filled 

 only with air, the two portions would interfere with each 

 other, and produce effects of colour according to the gene- 

 ral law. The ratio of the velocities, in water and in air, 

 IS that of 3 to 4; the fringes ought therefore to ap- 

 pear where the thickness is 6 times as great as that which 

 corresponds to the same colour in the common case of thin 

 plates; and upon making the experiment with a plain glass 

 and a lens slightly convex, I found the sixth dark circle 

 actually of the same diameter as the first in the new frin- 

 ges. The colours are also very easily produced, when but- 

 ter or tallow is substituted for water; and the rings then 

 become smaller, on account of the greater refractive'densify 

 of the oils : but, when water is added, so as to fill up the 

 mterstices of the oil, the rings are very much enlarged ; for 

 here the difference only of the velocities in- water and in 



