The Sun.
London, England, 1826-10-03 00:00:00

TUESDAY EVENING, OCTOBER 3, 1826.

SCIENCE.

No class of phenomena are more beautiful or inte-
resting than those which are presented in the analysis
of light into its component primary tints by, the prism.
The experiments are of so simple and familiar a nature
that the exhibition of these beautiful appearances forms
a ready and instructive amusement for a leisure hour
to any one who is possessed of a prism. Of late years
several new facts have been elicited on this subject ;
and we have no doubt many of our readers may be
interested in having a brief notice of the mode in
which this popular experiment is easily varied, so as
to shcw one or two curious results.

To exhibit the solar spectrum in perfection it is
necessary to have the room completely darkened,
whilst the sun shines directly upon the closed window,
and a ray is admitted through a small hole. The'
spectrum being thrown on the opposite wall, it will be
observed that the red end terminates very indistinctly.
Let, however, a mark be made at what would be
fairly called its furthest boundary. Then procure
a small piece of that sort of purplish glass, of which
sugar basons, &c. are made r---(it is to be observed, that
it is' not every kind that will do, as there is some dif-
ference in the shade of colour, but in trying a few
successively, some will not fail to answer :)---now,
looking through this glass, at the red boundary of
the 'spectrum, there will appear very distinctly ex-
tending- to the distance of half an inch or more be-
yond the mark before made, some very fine deep red
light. This, under ordinary circumstances, is render-
ed invisible from the proximity of brighter colours,
which produce a glare in the eye; the deep coloured
glass intercepts these, and allows the extreme red rays
to become visible. This is the discovery of Mr.
Herschel ; mid these rays are of peculiar interest in
reference to several points of science. In the first
place, as they form a clear well defined band of light,
they afford a fixed point in measuring the deviations
of the different rays, and thus learning tlke dispersive
powers of different species of glass, &c. so essential
to the perfection of achromatic telescopes. In the next
this. discovery explains a difficulty which had long
perplexed philosophers. The late Sir W. Herschel,
measured the relative degrees of heat produced in the
different rays of the spectrum, and found the greatest
effect produced in the extreme red as ordinarily visible ;,
he was however much surprised to find, that when
the liulb of the thermometer was placed at about half
an inch beyond the visible boundary, it rose still higher.
This observation was verified by others, and by the
fact above stated ; the explanation is obvious. The
effect in'both cases depends upon the nature of the
glass of which the prism is made. Some sorts of glass
disperse the rays differently, and throw the maxi-
mum point of heat to. .a difFersau position ; but with
good flint glass the phenomena are always as above
described.

We have spoken of the conditions which are essen-
tial for the most complete exhibition of the pheno-
mena; they may, however, be rendered visible
with less trouble ; if we merely look at the sun,
(without the necessity of darkening the room, &c.),
through the prism, and guard the eye with the coloured
glass, the appearance of the extreme red rays is very
distinct ; and if any other coloured glass be substituted
they become invisible.

In like manner the moon or any other luminous
object may be viewed. In some kinds of light these
rays will be found not to exist, and in general the inter-
position of glasses of different colours will produce a
number of interesting changes in the appearance of
the spectrum. These have been fully detailed and
examined by Mr. Herschel, in a paper published in
the "Edinburgh Transactions," vol. 9.

Another•not less interesting set of phenomena may
be observed in the following manner. In the upper
part of a window, looking against the sky, and on a
day when it is covered with white fleecy clouds, fix a
sheet of pasteboard, in which has been made an hori-
zontal slit, about 1-20th inch broad, and three or four
inches long, then retiring to the opposite part of the
room look at this slit through the prism, when a
spectrum will be seen exhibiting all the colours in the
form of a band or stripe of the same breadth as the
length of the slit ; to give it the greatest effect the room
should be darkened. If now the eye be attentively
fixed on any part of the coloured stripe, there will be-
come visible a number of dark and bright lines, suc-
ceeding each other alternately, crossing the spectrum
parallel to its ends and to the slit. These exist in all

party from the red to the violet end.
_ln the former

_••_.
part they occur closer together; and throughout the
whole length, they are not disposed with any regu-
larity. There are two or three among them darker
and broader than any of the rest ; some are extremely
faint, and others comparatively brilliant.
These lines were originally observed by Dr. Wol-
laston, (Phil. Trans. 1802.) but he distinguished
only a few of them ; he noticed also the very differ-
ent appearances presented when the same experiment
is tried with the light of a candle. This is partieu-
lady curious if we examine it at the blue part of the
flame, showing a separation of the colours with wide
intervals. •

'M. Frauenhofer, of Munich, who possesses a ve.
miller art in the manufacture of flint xlass, has tried
similar experiments with some of his incomparable
prisins, , and has succeeded in exhibiting a vastly
greater number of these lines than were before
seen. He counted upwards of stlo of them. Ile also
found that their relative positions varied considerably,
according to the nature of the refracting medium em-
pin. I
ployed ; he observed them in several other sorts of
light. In some of the brighter fixed stars there are
lines altogether different from those in the solar spec-
lines
; as also
acne
pe in
aspris
oflec the tr
sun's light.l
Engliih flint glass from 15
vi a:le i t
a he
goodtn. the planets the
to 20 ,lines tray be counted, when the experiment is
conducted as above described. In the light of the moon
be discerned, but the
similarly transmitted, according to Mr. Powell's ob-
servations, about six lines may
faint ; these correspond exactly in Position
spectrum. We
al lines in the solar
‘bs:.:ichltoliiel%et'ehiesnO'petrrllilelcOirpy has yet been proposed for explaining
the production of these lines.

A white or dove-colour robin (a very rare bird) was
caught in a net at Brighton on Dionday last.

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