440 ON A NEW SPECIES OF COLOURED FRINGES 



I now took three plates of parallel glass, that gave the co- 

 loured fringes when any two of them were put into the proper 

 position. When the third plate was placed either before the 

 other two, or between them, or behind them, it did not in the 

 least degree affect the fringes which they produced. When it 

 was placed, however, in such a position as to form a new re- 

 flected image, this image was also crossed by the coloured 

 fringes. 



When the third piece of parallel glass was cemented with 

 Canada balsam upon the face of the Jirst plate, or upon the 

 back of the second plate, the fringes disappeared. When the 

 interval between the two plates was filled with water, or with 

 Canada balsam, the fringes were very faint, though distinctly 

 perceptible. Hence it follows, that the production of the fringes 

 depends upon the action of all the four surfaces of the two plates 

 of parallel glass. 



All the preceding experiments were made with plates which 

 were cut out of the same piece of glass, and had therefore the 

 same thickness. I now tried plates of different thicknesses, 

 both when ground parallel, and when cut from common plate 

 o-lass ; but I could never render the coloured fringes visible, 

 unless when the glass was parallel, and exactly of the same 

 thickness in both plates. I also tried plates of topaz, of equal 

 thicknesses, and plates of sulphate of lime ; but though I used 

 pieces of various thicknesses, I have never succeeded in ma- 

 king them exhibit the coloured fringes, owing, perhaps, to the 

 imperfect flatness of their surfaces. 



In order to ascertain if the magnitude of the fringes depend- 

 ed on the thickness of the glass plates, I procured a piece of 

 parallel crown glass -r^oth of an inch thick, and compared the 

 fringes which it produced, at an inclination of 2' 20', and 

 at a vertical incidence, with those produced by another piece 

 of glass VoVsth of an inch thick. In the first case, the cir- 

 cular 



