LIGHT. 
is that some rays, at their incidence, are in 
fits of easy reflection, and others of easy 
transmission. For tire properties of reflect- 
ed liirht, see Mirror, Optics, &c. 
Again, a ray of light passing out of one 
niediupi into another of different density, 
, and in its passage making an oblique angle 
with the surface tliat separates the mediums, 
will be refracted, or turned out of its direc- 
tion ; because the rays are more strongly 
attracted by a denser, than by a rarer 
medium. That these rays are not refracted 
by striking on the solid parts of bodies, but 
that this is effected without a real contact, and 
by the same force by which they are emitted 
and reflected, onjy exerting itself differently 
in different circumstances, is proved, in a 
great measure, by the same arguments by 
which it is demonstrated that reflection is 
performed without contact. 
When light is refracted by a prism, or 
other transparent body, it is divided into 
rays exciting the sensation of different 
colours ; namely, red, orange, yellow, green, 
blue, indigo, and violet. This is the enu- 
meration followed by Newton and others, 
which supposes seven rays refrangible in the 
above order, the red being least refrangible 
and the violet most so, and that the other 
tints are produced by mixture. The image 
formed by the different rays, thus separated, 
forms the solar spectrum. ' Dr. Wollaston 
has shewn, by looking through the prism at 
a narrow line of light, that the primitive 
colours are only red, green, blue, and 
violet. 
Heat and light are not present in corres- 
ponding degrees, in different parts of the 
solar sjicet! um ; for„ generally speaking, 
those rays illuminate most that have the 
least heating power. The rays in the centre 
of the spectrum have the greatest illumi- 
nating power, as may be ascertained by 
viewing, successively in each, a small body, 
such as the head of a common nail. It will 
be seen most distinctly in the light green, 
or deep yellow rays, and less plainly to- 
wards either extremity of the spectrum. 
The heating power of the rays follows a 
different order. If the bulb of a sensible 
thermometer be moved, in succession, 
through the differently coloured rays, it will 
be found to indicate the greatest heat in 
the red rays, next in the green, and so on, 
in a diminishing progression, to the violet. 
When tire thermometer is removed entirely 
out of the confines of the red rays, but with 
its ball still in the line of the spectrum, it 
rises even higher than in tlie red rays ; and 
continues to rise till removed half an inch 
beyond the extremity of the red ray. The 
ball of tlie thermometer employed for this 
purpose should be extremely small, and 
should be blackened with Indian ink. An 
air thermometer is better adapted than a 
mercurial one, to exhibit the minute change 
of temperature that ensues. These invisible 
heat-making rays may be reflected by the 
mirror, and refracted by the lens, exactly 
in the same manner as the rays of light. 
Beyond the confines of the spectrum on 
the other side, viz, a little beyond the violet 
ray, the thermometer is not affected ; but 
in this place it is remarkable that there are 
also invisible rays of a different kind, which 
exert all the chemical effects of the rays of 
light, and even with greater energy. One 
of the chemical properties of light is, that it 
speedily changes from white to black the 
fresh precipitated muriate of silver. This 
effect is produced most rapidly by the direct 
light of the sun ; and the rays, as separated 
by the prism, have this property in various 
degrees. The blue rays, for example, effect 
a change of the muriate of silver in fifteen 
seconds, which the red require twenty 
minutes to accomiilish ; and, generally speak- 
ing, the power diminishes as we recede from 
the violet extremity. But entirely out of 
the spectrum, and beyond the violet rays, 
the effect is still produced. Hence it ap- 
pears that the solar beams consist of three 
distinct kinds of rays ; of those that excite 
heat, and promote oxydation j of illuminat- 
ing rays ; and of de-oxydizing rays. A strik- 
ing illustration of the different power of these 
various rays, is furnished by their effect on 
phosphorus. In the rays beyond the red 
extremity, phosphorus is heated, smokes, 
and emits white fumes ; but these are pre- 
sently suppressed on exposing it to the de- 
oxydizing rays which lie beydnd tlie violet 
extremity. 
There is an exception, however, as stated 
by Dr. Wollaston, to the de-oxydizing 
power of the rays above-mentioned. The 
substance, termed gum-guiacum, has the 
property, when exposed to the light, of 
changing from a yellowish colour to green ; 
and this effect he has .ascertained to be con- 
nected with the absorption of oxygen. Now 
in the most refrangible rays, which would 
fall beyond the violet extremity, he found 
that this substance became green, and was 
again changed to yellow by the least refran- 
gible. This is precisely the reverse of what 
happens to muriate of silver, which is 
blackened, or de-oxydized, by the most 
