"PRODUCED BY TWO PLATES OF GLASS OP EQUAL THICKNESS. 439 



When the fringes on the second image were inclined to the 

 right, those on the first image were inclined to the left ; so that 

 both in point of position and magnitude, the two sets of frin- 

 ges follow a different law. 



The preceding measures of the magnitude of the fringes at 

 different obliquities, were not taken with that accuracy which 

 is necessary for determining the law of their variation. I have 

 made numerous experiments for this purpose ; but when the 

 angle of incidence is considerable, there is always such a de- 

 gree of distortion in the fringes, and such a perceptible varia- 

 tion in their magnitude, from the slightest change in the posi- 

 tion of the eye, that I found it quite impracticable to take mea- 

 sures in which any confidence could be placed. This difficulty 

 no doubt arises from the imperfect flatness of the surfaces of 

 the plates of glass ; and I fear that even our best artists are 

 not capable of producing better plates than those which I used 

 in the preceding experiments. The following measures may 

 be considered as tolerably correct. 



The inclination of the plates was not the same as in the pre- 

 ceding experiments : 



If the two parallel plates are placed at ani/ distance whatever, 

 and the preceding experiments repeated, the fringes will be 

 found to suffer no change either in their magnitude or direc- 

 tion, 



3 K 2 I 



