110 PROFESSOR AIRY ON THE CALCULATION OF 



rectangular hole, I am inclined to think that, when the apertures are 

 centrally opposite, the same investigation will apply well to it. 



I may now without impropriety mention the circumstances which 

 induced me to make this investigation. 



In Newton's Optics, Book iii. Observation 6, Newton describes in 

 very striking language the effect of narrowing a slit on which the 

 sun-light fell after having passed through a hole a quarter of an inch 

 in diameter. He states that when the breadth of the slit was about 

 —t\\ of an inch, the illumination on the screen was interrupted by 

 a black shadow in the middle. It is certain, theoretically and prac- 

 tically, that if the experiment had been made in Fresnel's method the 

 center would be the brightest part. It seemed therefore worth while 

 to ascertain, by the best kind of investigation that svich an un- 

 manageable case admits of, whether the size of the external hole 

 could account for the dark shadow. From consideration of the form 

 of the illumination in the second case above, it appears certain that 

 it could not. The only resource (which the dullness of the weather 

 at that time denied me) was to repeat the experiment. This I have 

 now done three separate times in the presence of as many different 

 persons : I have used both parallelogrammic and circular holes of dif- 

 ferent sizes (the largest circular hole being ^inch in diameter) and 

 have sometimes diminished the aperture to as little as j^ inch (by 

 estimation). The distances have been 30 inches each, which appear 

 to have been the distances in Newton's experiments. In every in- 

 stance the center has been bright. I can account for this inaccuracy 

 in Newton's observation only by supposing that his eye was in such 

 a state as not to recover from the sudden impression which is pro- 

 duced by rapidly diminishing the central light on the screen (which 

 makes it for an instant appear black), and by referring to his candid 

 avowal in the Advertisement, that " the third book and the last pro- 

 " position of the second were put together out of scattered papers," 

 and that " The subject of the third book I have also left imperfect, 

 " not having tried all the experiments which I intended when I was 



