156 EXPERIMENTS AND INVESTIGATIONS 



that, on the other hand, c b and d a are affected when the 

 edges are placed parallel to these two sides of the rays ; and 

 thus the connection of the fringes in question, with the 

 preceding action of which disposed and polarized, is clearly 

 proved. 



4. It is an obvious extension and variation of this experi- 

 ment both to apply edges parallel to the first and disposing 

 edges, and also to apply edges at right angles to their 

 direction ; and important results follow from this experiment. 

 But until a more minute examination of the phenomena with 

 accurate admeasurements can be had, I prefer not entering 

 on this subject further than to say, that the extreme difficulty 

 of obtaining fringes or images at once from the edges parallel 

 to the first two, and from edges at right angles to these, 

 indicates an action not always at right angles to the bending 

 body, but whether conical or not I have not hitherto been 

 able to ascertain. That the first body only disposes and 

 polarizes in one direction is certain. But it seems difficult to 

 explain the effect of the first two edges in preventing the 

 fringes or images from being made by the second at right 

 angles to those formed by the first two edges, if no lateral 

 action exists. One can suppose the approaching of those two 

 first edges to make the fringes narrower and narrower than 

 those which the second two edges form when placed at right 

 angles to the first. But this is by no means all that happens. 

 There is hardly any set of fringes at all formed at right 

 angles to the first set (parallel to the first two edges) when 

 the first two are approached so near each other as greatly to 

 distend the disc. 



5. I reserve for future inquiry also the opinion held by 

 Sir I. NEWTON, that the different homogeneous rays are acted 

 upon by bodies at different distances, this action extending 

 furthest over the least refrangible rays. He inferred this 

 from the greater breadth of the fringes in those rays. 



It is in my apprehension, though I once held a different 

 opinion,* not impossible to account for the difference of the 

 * Philosophical Transactions, 1797. 



