Encyclopaedia Britannica > Volume 15, NIC-PAR

S5.
Experi-
57
M. Mai-
ran’s tlie-
ory.
of the plates which were a little more than one line
broad, though they were received at the distance o 72
feet 1 hut be could observe a difference ot shades in
those of the globes, taken at the same distance, though
they were 2t lines in diameter.
In order to explain the colours at the edges of these
shadows, he threw some of the shadows upon others.
„ - . He threw the gleam of light, which always intervened
meStswith between the colours and the darker part ot the shadow,
a mixture upon different parts of other shadows j and observed,
of coloured ^jla^ -when it fell upon the exterior penumbra made a
shadows. anot’ber neetUe, it produced a beautiful sky blue colour,
almost like that which was produced by two blue co¬
lours thrown together. When the same gleam oi light
fell upon the deeper shadow in the middle, it produced
a red colour. r i-
He placed two plates of iron, each three or ,our lines
broad, at a very small distance: and having placed
them in the rays of the sun, and received their shadows
at the distance of 15 or 20 feet from them, he saw no
light between them but a continued shadow, in the mid¬
dle of which were some parallel streaks of a lively pui-
ple, separated by other black streaks j but between them
there were other streaks, both of a very faint green, and
also of a iiale yellow. . . A)r
The subject of inflection was next investigated byJYJ.
Mairan : but he only endeavoured to explain the facts
which were known, by the hypothesis of an atmosphere
surrounding all bodies ; and consequently making two
reflections and refractions of the light that falls upon
them, one at the surface of the atmosphere, and the
other at that of the body. This atmosphere he suppo¬
sed to be of a variable density and refractive power,
rg like the atmosphere.
Discoveries M. Du Tour thought the variable atmosphere super-
cf M. Du fiuous? and attempted to account for all the phenomena
by an atmosphere of an uniform density, and of a less
refractive power than the air surrounding all bodies.
Only three fringes had been observed by preceding
authors, but M. Du Tour was accidentally led to ob¬
serve a greater number of them, and adopted from Gri¬
maldi the following ingenious method of making them
all appear very distinct.
He took a circular board ABED, (fig. 9.) 13 inches
in diameter, the surface of which was black, except at
the edge, where there was a ring of white paper about
three lines broad, in order to trace the circumference of
a circle, divided into 360 degrees, beginning at the point
A, and reckoning 180 degrees on each hand to the
point E ; B and D being each of them placed at 90 de-
o-rees. A slip of parchment 3 inches broad, and dispo¬
sed in the form of a hoop, was fastened round the board,
and pierced at the point E with a square hole, each side
being 4 or 5 lines, in order to introduce a ray of the
sun’s fight; and in the centre of the board C, he fixed
a perpendicular pin about y ot a line in diameter.
This hoop being so placed, that a ray of light enter¬
ing the chamber, through a vertical cleft of 2-q lines in
length, and about as wide as the diameter of the pin,
went through the hole at E, and passing parallel to the
plane of the board, projected the image of the sun and
the shadow of the pin at A. In these circumstances
he observed, 1. That quite round the concave surface of
this hoop, there were a multitude of coloured streaks j
but that the space m A 71, of about 18 degrees, the
4
Tour.
OPTICS.
middle of which was occupied by the image of the sun, Histaiy.
was covered with a faint light only. 2. The order of1 '
the colours in these streaks was generally such that the
most refrangible rays were the nearest to the incident
ray EGA so that, beginning from the point A, the
violet was the first and the red the last colour in each ot
the streaks. In some of them, however, the colours
were disposed in a contrary order. 3. The image of the
sun, projected on each side of the point A, was divided
by the shadow of the pin, which was bordered by two
luminous streaks. 4. The coloured streaks were nar¬
rower in some parts of the hoop than others, and gene-
rally decreased in breadth in receding from the point A.
c Among these coloured streaks, there were sometimes
others which were white, 1 or lines in breadth,
which were generally bordered on both sides by a streak
of orange colour.
From this experiment he thought it evident, that the
rays which passed beyond the pin were not the only ones
that were decomposed, for that those which were ic-
flected from the pin were decomposed also, whence he
concluded that they must have undergone some refrac¬
tion. He also imagined that those which went beyond
the pin suffered a reflection, so that they were all affect¬
ed in a similar manner. . ™ . 59 f
In order to give some idea of his hypothesis, M. Du Account oi
Tour shows that the ray o 6, fig. 10. after being refract- ^
ed at 6, reflected at r and w, and again refracted at ,9 ^
and t, will be divided into its proper colours ; the least
refrangible or the red rays issuing at .r, and the most
refrangible or violet at y. Those streaks in which the
colours appear in a contrary order he thinks are to be
ascribed to inequalities in the surface of the pin.
The coloured streaks nearest the shadow of the pin,
he supposes to be formed by those rays which, entering
the atmosphere, do not fall upon the pin •, and, without
any reflection, are only refracted at their entering and ^
leaving the atmosphere, as at & and rw, fig. i t. In
this case, the red or least refrangible rays will issue at
and the violet at «.
Plate
CCCLXXVI.
fig. p.
’ To distinguish the rays which fell upon the hoop in
any particular direction, from those that came in any
other, he made an opening in the hoop, as at P, fig. 9*
by which means he could, with advantage, and at any
distance from the centre, observe those rays unmixed
with any other.
To account for the coloured streaks being larger next
the shadow of the pin, and growing narrower to the
place where the light was admitted, he shows, by fig. 12. *£•
that the rays a b are farther separated by both the re¬
fractions than the rays c d.
Sometimes M. Dn Tour observed, that the broader
streaks were not disposed in this regular order 5 but then ^
he found, that by turning the pin they changed their
places, so that this circumstance must have been an ac¬
cidental irregularity in the surface of the pin.
The white streaks mixed with the coloured ones he
ascribes to small cavities in the surface of the pin *, tor
they also changed their places when the pin was turned
upon its axis. „
He also found, that bodies of various kinds, and ot
different sizes, always produced fringes of the same di¬
mensions. _ . .
Exposing two pieces of paper in the beam ot hgnt,
so that part of it passed between two planes formed by
A them,