PRODUCED BY TWO PLATES OF GLASS OF. EQUAL THICKNESS. 441 



cular image was crossed by Jive fringes, and in the second 

 case with seven fringes : But 



i^ : i??- : : 5 : 7 nearly. 

 1000 1000 



In another experiment, I found, from a mean of five measure- 

 ments, that the thickest of these pairs of plates produced frin- 

 ges each of which had a breadth of U' lO", when the inclina- 

 tion of the plates was l" 5^'. Now the other pair of plates 

 gave fi-inges 13' 3" broad, at an inclination of 2° 20", which, 

 •rives 15' 29" for their breadth at an angle of 1° 58', and 



I?L . i_6-?-::ll'10":15'30". 

 1000 ■ 1000 



Hence the magnitudes of the fringes are inversely as the thick- 

 nesses of the plates tihich produce them, at a given inclination ; 

 and in general the magnitudes of the fringes are in the compound' 

 inverse ratio of the thickness of the plates, and of their angle of 

 inclination. 



Hitherto we have supposed the glass plates to be placed be- 

 tween the eye and tlie luminous object, so that only the 2d, 

 4th, and 6th reflected images were seen. When the eye is 

 placed between the plates and the luminous object, so as to 

 perceive the 1st, 3d, and 5th, reflected images, the coloured 

 frino'es are also seen, having the same characters as those alrea- 

 dy noticed. 



The phenomena which have been described are equally pro- 

 duced when the fringes are formed by polarised light, and they 

 do not suffer the least change when examined by doubly re- 

 fracting or doubly polarising crystals. 



When the eye is placed at a considerable dis.ance, either 

 before or behind the glass plates, all the fringes have a very 



distorted 



