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Sorry for the late posting. I changed my posting email address
and did not notify the list, so the original did not get through.
On Thu, 30 Oct 2003, Ludwik Kowalski wrote:
But was the real experiment
performed by using the sun or a flame? That is the question.
Also was it a set of slits or a set of pinholes?
"Exper. 1. 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 position as
to reflect the sun's light, in a direction nearly horizontal,
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 the shadow, in its way towards
the fringes. Nor was it for want of a sufficient intensity of
light, that one of the two portions was incapable of producing
the fringes alone; for, when they were both uninterrupted, the
lines appeared, even if the intensity was reduced to one-tenth or
one-twentieth."
"The Bakerian Lecture: Experiments and Calculations Relative to
Physical Optics," Thomas Young, _Philosophical Transactions of
the Royal Society of London_, Vol. 94 (1804), 1-16. (Read
November 24, 1803.)
--
Stephen
stephen@speicher.com
Ignorance is just a placeholder for knowledge.
Printed using 100% recycled electrons.
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