" On the Inflexion of Light" 433 



tainly of a nature deserving the most attentive examination, 

 and supported by good computations of the theoretical results, 

 altogether evincing much research and skill. If I have been 

 compelled to differ from him as to the comparison instituted 

 with earlier experiments, and in the actual results observed in 

 the last case, I hope that he will regard these remarks in no 

 other light than as conspiring with his own researches in the 

 common cause of scientific truth. 



P.S. — In immediate connexion with the foregoing remarks, 

 I have now to add, that I have received from Prof. Airy the 

 following statement relative to these experiments ; and I gladly 

 avail myself of his permission to lay it before the readers of 

 this Journal. 



" 1 have repeated several times Newton's experiment on 

 knife-edges, placed at a very small distance apart. As my ob- 

 ject was to verify Newton's observations, and to verify Fresnel's 

 theory, as far as calculations can be applied, I carefully avoided 

 the use of curved edges, which introduce insurmountable dif- 

 ficulties into the mathematical investigation. My aperture 

 was made by two rectilinear cheeks, adjustible by a screw. 

 The edges of the cheeks are very truly worked. I have tried 

 holes for admitting the sun's light into the room, of various 

 breadths, from about £ inch (which was the breadth of New- 

 ton's hole in this experiment,) downwards. The aperture be- 

 tween the cheeks has in general been varied through all the 

 values between jq inch and t ^q inch or less (the latter 

 measure is by estimation). My distances from the hole to the 

 aperture, and from the aperture to the screen, have always 

 been 30 inches ; some of the observations have been made by 

 myself alone, and others in the presence of another person. 

 In every instance the centre of the image thrown on the screen 

 has been the brightest part. There is one circumstance which 

 would easily account for a careless observer supposing that 

 the centre was dark : when the aperture is large ( j\j inch for 

 instance), the centre is very bright. If now the aperture is 

 suddenly contracted, and the central light consequently much 

 diminished, the centre seems for a short time black. But this 

 is merely the nervous effect of surprise on the eye ; for on al- 

 lowing the eye to rest for a few seconds, it becomes evident 

 that the centre is brightest. If instead of contracting the aper- 

 ture from a wide opening to a narrow one, it be gradually 

 opened from a very narrow interval to a wide one, it is evi- 

 dent to the eye through the whole change that the centre is 

 bright. 

 Third Series. Vol. 2. No. 12. June 1833. 3 K 



