Astronomical and Nautical Collections, 453 



steel, placed very near to each other, their vertical edges, 

 which were perfectly rectilinear, being ground to an edge 

 at one part, and rounded at the other, the sharp edge of 

 each being opposite to the rounded edge of the other ; the 

 sharp edge being to the right, for example, at the upper 

 part, and to the left at the lower. In this manner, how- 

 ever small the difference might have been in the actions of 

 the two parts on the light, it could not but have been 

 observed at the point of junction of the two corresponding 

 portions of the enlightened space, and more especially at 

 the points of junction of the fringes, which accompanied it, 

 and which would have been interrupted where the sharp 

 and the round edges were abruptly interchanged. But, 

 upon observing these stripes with great attention, 1 could 

 discover in them nothing like interruption or bending at 

 any part of their length ; they were straight and continued, 

 exactly as if the plates had been placed with their similar 

 portions opposed to each other. 



Several years before this time, Malus and Berthollet, in 

 making experiments on diffraction, with plates composed of 

 two parts of different nature, one of ivory, and the other of 

 metal, for example, had ascertained from the situation of 

 the fringes, that the diffractive effects of these different sub- 

 stances were the same ; and though the observations of these 

 celebrated persons could not possess all the accuracy of the 

 measurements obtained by the micrometer, in the way which 

 I have described, they were still sufficient to demonstrate, 

 that if the difference of the nature of the substances had 

 some un perceived influence on the direction of the rays, 

 it was still much less than would be expected from the 

 great difference of the refractive and reflective powers of 

 the substances employed, if we supposed the inflection of 

 light to be produced by attractive or repulsive forces 

 acting on the luminous particles. [Dr. Young had long 

 before made some similar observations, in consequence of 

 which he asserted, in his Lectures, published in 1807, that 

 ** there is some improbability in supposing that bodies of 

 different forms, and of various refractive powers, should 

 possess an equal force of inflection, as they appear to do, in 

 the production of these effects." P* 458.] 



