s Dr. Young’s Experiments and Calculations 
the sun shines, and without any other apparatus than is at hand 
to every one. 
Exper. i. I made a small hole in a window-shutter, and 
covered it with a piece of thick paper, which I perforated with a 
fine needle. For greater convenience of observation, I placed a 
small looking glass without the window-shutter, in such a po- 
sition as to reflect the sun’s light, in a direction nearly hori- 
zontal, upon the opposite wall, and to cause the cone of diverging 
light to pass over a table, on which were several little screens of 
card-paper. I brought into the sunbeam a slip of card, about 
one-thirtieth of an inch in breadth, and observed its shadow, 
either on the wall, or on other cards held at different distances. 
Besides the fringes of colours on each side of the shadow, the 
shadow itself was divided by similar parallel fringes, of smaller 
dimensions, differing in number, according to the distance at 
which the shadow was observed, but leaving the middle of the 
shadow always white. Now these fringes were the joint effects 
of the portions of light passing on each side of the slip of card, 
and inflected, or rather diffracted, into the shadow. For, a little 
screen being placed a few inches from the card, so as to receive 
either edge of the shadow on its margin, all the fringes which 
had before been observed in the shadow on the wall immediately 
disappeared, although the light inflected on the other side was 
allowed to retain its course, and although this light must have 
undergone any modification that the proximity of the other edge 
of the slip of card might have been capable of occasioning. 
When the interposed screen was more remote from the narrow 
card, it was necessary to plunge it more deeply into the shadow, 
in order to extinguish the parallel lines; for here the light, 
diffracted from the edge of the object, had entered further into 
