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



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

 case with seven fringes : But 



121 168 -." , 



— : : 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 11' 10", when the inclina- 

 tion of the plates was 1° 58'. Now the other pair of plates 

 gave fringes 13' 3" broad, at an inclination of 2 Q 20", which 

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



2_2J_ J_68_ iriQr i 5 ' 30 " m 

 1000 1000 



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

 nesses of the plates which 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 the 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 

 fringes 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 



