PRODUCED BY TWO PLATES OF GLASS OF EQUAL THICKNESS. 437 



the bright and the reflected image be varied by a gentle mo- 

 tion of one of the plates, the coloured fringes will be found to 

 increase in breadth as the inclination of the plates is diminish- 

 ed, and to diminish as the inclination of the plates is increa- 

 sed. 



In order to determine the law according to which the mag- 

 nitude of the fringes varies, I employed two plates of paral- 

 lel glass -Vs^ths of an inch thick, and obtained the following 

 measures for the fringes which crossed the image that had 

 suffered two reflexions between the plates. The pencil of 

 light was incident nearly in a vertical direction upon the 

 first plate. 



Inclination of the Plates. Angular breadth of each Fringe. 



1° 11' 26' 50" 



2 20 13 3 



5 36 5 41 



Now since 5° 36' : 26'50" : : 1° 11' : 5' 40*, and since 



5 36 : 13 3 : : 2 20 : 5 27 , it follows, that 



the breadth of the fringes is inversely as the inclination of the 

 plates. 



Owing to the rapid diminution of the fringes, by increasing 

 the angle formed by the plates, I could not with any degree 

 of accuracy determine their breadth at greater angles of incli- 

 nation ; and therefore it still remains to be ascertained whe- 

 ther it varies with the sine, tangent, or secant of the angles. 



If the light of the circular object, instead of falling perpen- 

 dicularly upon the plates, is incident at different obliqui- 

 ties, so that the plane of incidence is at right angles to the com- 

 mon section of the plates, no fringes are visible across any of the 

 images. But if the plane of incidence is parallel to the common 

 section of the plates, the reflected images increase in brightness 

 with the obliquity of incidence, and the coloured fringes be- 

 come more vivid. When the angle of incidence increases 

 from Q to 90°, the images that have suffered the greatest num- 



Vol. VII. P. II. 3 K ber 



