144 EXPERIMENTS AND INVESTIGATIONS 



the greater proximity of the smaller fringe, altered the pro- 

 portions. 



PROPOSITION VIII. 



It is proved by experiment that the inflexion of the second 

 body makes broader fringes or images than its deflexion after 

 the inflexion of the first body ; and also that the inflecto- 

 deflexion fringes decrease, and the deflecto-inflexion fringes 

 increase with the distance from the direct rays. 



Exp. 1. It must be observed that when we examine the 

 fringes (or images) made by the second edge deflecting the 

 rays which the first had inflected, we can see the effects of 

 the disposition communicated to the rays at a much greater 

 distance of the second edge from the first, than we can 

 perceive the effects of that disposition upon the inflexion by 

 the second edge of the rays deflected by the first. Indeed 

 we only lose the fringes thus made by deflexion, in con- 

 sequence of their becoming so minute as to be imperceptible 

 to our senses. But it is otherwise with the fringes or 

 images made by the second edge inflecting the rays which 

 the first had deflected. These can only be seen when the 

 second edge is near the first, because the rays cannot pass on 

 so as to form the images on the chart, if the second is distant 

 from the first. The pencils diverge both by the deflexion 

 and by the inflexion of the first edge. But we can always, 

 when the inflected rays pass too far from the second edge, 

 bring this so near them as to act on them, whereas we in so 

 doing intercept the deflected rays. However, after this is 

 explained, we find no difficulty in examining the effects of 

 the inflexion by the second edge, only we must place it near 

 the first, and thus we have two sets of fringes, one ex- 

 tending into the shadow of the first edge at an inch distance 

 between the two edges; but at an inch and three-fourths, 

 nay, at two inches, or even more, this experiment can well 

 be made. 



Exp. 2. At these distances I examined repeatedly the 

 comparative breadths of the two sets. In fig. 16, ab is the 



