_Mistake of Newton in supposing that the Improvement of Refracting
Telescopes was hopeless—Mr. Hall invents the Achromatic Telescope—
Principles of the Achromatic Telescope explained—It is re-invented
by Dollond, and improved by future Artists—Dr. Blair’s Aplanatic
Telescope—Mistakes in Newton’s Analysis of the Spectrum—Modern
Discoveries respecting the Structure of the Spectrum._
The new doctrines of the composition of light, and of the different
refrangibility of the rays which compose it, having been thus
established upon an impregnable basis, it will be interesting to take a
general view of the changes which they have undergone since the time
of Newton, and of their influence on the progress of optical discovery.
There is no fact in the history of science more singular than that
Newton should have believed that all bodies produced spectra of equal
length, or separated the red and violet rays to equal distances when
the refraction of the mean rays was the same. This opinion, unsupported
by experiments, and not even sanctioned by any theoretical views,
seems to have been impressed upon his mind with all the force of an
axiom.[16] Even the shortness of the spectrum observed by Lucas did
not rouse him to further inquiry; and when, under the influence of
this blind conviction he pronounced the improvement of the refracting
telescope to be desperate, he checked for a long time the progress of
this branch of science, and furnished to future philosophers a lesson
which cannot be too deeply studied.
In 1729, about two years after the death of Sir Isaac, an individual
unknown to science broke the spell in which the subject of the spectrum
had been so singularly bound. Mr. Chester More Hall, of More Hall
in Essex, while studying the mechanism of the human eye, was led to
suppose that telescopes might be improved by a combination of lenses
of different refractive powers, and he actually completed several
object-glasses upon this principle. The steps by which he arrived at
such a construction have not been recorded; but it is obvious that
he must have discovered what escaped the sagacity of Newton, that
prisms made of different kinds of glass produced different degrees of
separation of the _red_ and _violet_ rays, or gave spectra of different
lengths when the refraction of the middle ray of the spectrum was the
same.
[Illustration: _Fig. 6._]
In order to explain how such a property led him to the construction
of a _telescope without colour_, or an _achromatic telescope_, let us
take a lens LL of _crown_ or _plate_ glass, whose focal length LY is
about twelve inches. When the sun’s rays SL, SL fall upon it, the _red_
will be refracted to R, the _yellow_ to Y, and the _violet_ to V. If we
now place behind it a concave lens _ll_ of the same glass, and of the
same focus or curvature, it will be found, both by experiment and by
drawing the refracted rays, according to the rules given in elementary
works, that the concave glass _ll_ will refract the rays LR, LR into
LS′, LS′, and the rays LV, LV into LS′, LS′ free of all colour; but as
these rays will be parallel, the two lenses will not have a focus, and
consequently cannot form an image so as to be used as the object-glass
of a telescope. This is obvious from another consideration; for since
the curvatures of the convex and concave lenses are the same, the two
put together will be exactly the same as if they were formed out of a
single piece of glass, having parallel surfaces like a watch-glass,
so that the parallel rays of light SL, SL will pass on in the same
direction LS′, LS′ affected by equal and opposite refractions as in a
piece of plane glass.
Now, since the convex lens LL separated the white light SL, SL into
its component coloured rays, LV, LV being the extreme violet, and LR,
LR the extreme red; it follows that a similar concave lens of the same
glass is capable of uniting into white light LS′, LS′ rays, as much
separated as LV, LR are. Consequently, if we take a concave lens _ll_
of the same, or of a greater refractive power than the convex one, and
having the power of uniting rays farther separated than LV, LR are, a
less concavity in the lens _ll_ will be sufficient to unite the rays
LV, LR into a white ray LS′; but as the lens _ll_ is now less concave
than the lens LL is convex, the concavity will predominate, and the
uncoloured rays LS′, LS′ will no longer be parallel, but will converge
to some point O, where they will form a colourless or achromatic image
of the sun.
The effect now described may be obtained by making the _convex_ lens LL
of _crown_ or of _plate_ glass, and the _concave_ one of _flint_ glass,
or that of which wineglasses are made. If the concave lens _ll_ has a
greater refractive power than LL, which is always the case, the only
effect of it will be to make the rays converge to a focus more remote
than O, or to render a less curvature necessary in _ll_, if O is fixed
for the focus of the combined lenses.
Such is the principle of the achromatic telescope as constructed by Mr.
Hall. This ingenious individual employed working opticians to grind
his lenses, and he furnished them with the radii of the surfaces,
which were adjusted to correct the aberration of figure as well as of
colour. His invention, therefore, was not an accidental combination of
a convex and a concave lens of different kinds of glass, which might
have been made merely for experiment; but it was a complete achromatic
telescope, founded on a thorough knowledge of the different dispersive
powers of crown and flint glass. It is a curious circumstance,
however, in the history of the telescope, that this invention was
actually lost. Mr. Hall never published any account of his labours,
and it is probable that he kept them secret till he should be able to
present his instrument to the public in a more perfect form; and it
was not till John Dollond had discovered the property of light upon
which the instrument depends, and had actually constructed many fine
telescopes, that the previous labours of Mr. Hall were laid before the
public.[17] From this period the achromatic telescope underwent gradual
improvement, and by the successive labours of Dollond, Ramsden, Blair,
Tulley, Guinand, Lerebours, and Fraunhofer, it has become one of the
most valuable instruments in physical science.
Although the achromatic telescope, as constructed by Dollond, was
founded on the principle that the spectra formed by crown and flint
glass differed only in their relative lengths, when the refraction of
the mean ray was the same, yet by a more minute examination of the
best instruments, it was found that they exhibited white or luminous
objects tinged on one side with a green fringe, and on the other with
one of a claret colour. These colours, which did not arise from any
defect of skill in the artist, were found to arise from a difference in
the extent of the coloured spaces in two equal spectra formed by crown
and by flint glass. This property was called the _irrationality_ of
the coloured spaces, and the uncorrected colours which remained when
the primary spectrum of the crown glass was corrected by the primary
spectrum of the flint glass were called the _secondary_ or _residual
spectrum_. By a happy contrivance, which it would be out of place here
to describe, Dr. Blair succeeded in correcting this secondary spectrum,
or in removing the green and claret-coloured fringes which appeared in
the best telescopes, and to this contrivance he gave the name of the
_Aplanatic Telescope_.
But while Newton thus overlooked these remarkable properties of the
prismatic spectrum, as formed by different bodies, he committed some
considerable mistakes in his examination of the spectrum which was
under his own immediate examination. It does not seem to have occurred
to him that the relations of the coloured spaces must be greatly
modified by the angular magnitude of the sun or the luminous body, or
aperture from which the spectrum is obtained; and misled by an apparent
analogy between the length of the coloured spaces and the divisions
of a musical chord,[18] he adopted the latter, as representing the
proportion of the coloured spaces in every beam of white light. Had two
other observers, one situated in Mercury, and the other in Jupiter,
studied the prismatic spectrum of the sun by the same instruments,
and with the same sagacity as Newton, it is demonstrable that they
would have obtained very different results. On account of the apparent
magnitude of the sun in Mercury, the observer there would obtain a
spectrum entirely without _green_, having _red_, _orange_, and _yellow_
at one end, the _white_ in the middle, and terminated at the other
end with _blue_ and _violet_. The observer in Jupiter would, on the
contrary, have obtained a spectrum in which the colours were much more
condensed. On the planet Saturn a spectrum exactly similar would have
been obtained, notwithstanding the greater diminution of the sun’s
apparent diameter. It may now be asked, which of all these spectra are
we to consider as exhibiting the number, and arrangement, and extent
of the coloured spaces proper to be adopted as the true analysis of a
solar ray.
The spectrum observed by Newton has surely no claim to our notice,
merely because it was observed upon the surface of the earth. The
spectrum obtained in Mercury affords no analysis at all of the incident
beam, the colours being almost all compound, and not homogeneous, and
that of Newton is liable to the same objection. Had Newton examined his
spectrum under the very same circumstances in winter and in summer, he
would have found the analysis of the beam more complete in summer, on
account of the diminution of the sun’s diameter; and, therefore, we are
entitled to say that neither the number nor the extent of the coloured
spaces, as given by Newton, are those which belong to homogeneous and
uncompounded light.
The spectrum obtained in Jupiter and Saturn is the only one where
the analysis is complete, as it is incapable of having its character
altered by any farther diminution of the sun’s diameter. Hence we
are forced to conclude, not only that the number and extent of the
primitive homogeneous colours, as given by Newton, are incorrect; but
that if he had attempted to analyze some of the primitive tints in the
spectrum, he would have found them decidedly composed of heterogeneous
rays. There is one consequence of these observations which is somewhat
interesting. A rainbow formed in summer, when the sun’s diameter is
least, must have its colours more condensed and homogeneous than in
winter, when the size of its disk is a maximum, and when the upper or
the under limb of the sun is eclipsed, a rainbow formed at that time
will lose entirely the yellow rays, and have the green and the red in
perfect contact. For the same reason, a rainbow formed in Venus and
Mercury will be destitute of green rays, and have a brilliant bow of
white light separating two coloured arches; while in Mars, Jupiter,
Saturn, and the Georgian planet, the bow will exhibit only four
homogeneous colours.
From his analysis of the solar spectrum, Newton concluded, “that to
the same degree of refrangibility ever belonged the same colour, and
to the same colour ever belonged the same degree of refrangibility;”
and hence he inferred, that _red_, _orange_, _yellow_, _green_, _blue_,
_indigo_, and _violet_ were primary and simple colours. He admitted,
indeed, that “the same colours in specie with these primary ones may
be also produced by composition. For a mixture of _yellow_ and _blue_
makes _green_, and of _red_ and _yellow_ makes _orange_;” but such
compound colours were easily distinguished from the simple colours of
the spectrum by the circumstance, that they are always capable of being
resolved by the action of the prism into the two colours which compose
them.
This view of the composition of the spectrum might have long remained
unchallenged, had we not been able to apply to it a new mode of
analysis. Though we cannot separate the _green_ rays of the spectrum
into _yellow_ and _blue_ by the refraction of prisms, yet if we
possessed any substance which had a specific attraction for _blue_
rays, and which stopped them in their course, and allowed the _yellow_
rays to pass, we should thus analyze the _green_ as effectually as
if they were separated by refraction. The substance which possesses
this property is a purplish blue glass, similar to that of which
finger-glasses are made. When we view through a piece of this glass,
about the twentieth of an inch thick, a brilliant prismatic spectrum,
we find that it has exercised a most extraordinary absorptive action
on the different colours which compose it. The _red_ part of the
spectrum is divided into _two red_ spaces, separated by an interval
entirely devoid of light. Next to the inner red space comes a space
of bright _yellow_, separated from the red by a visible interval.
After the yellow comes the _green_, with an obscure space between
them, then follows the _blue_ and the _violet_, the last of which has
suffered little or no diminution. Now it is very obvious, that in this
experiment, the blue glass has actually absorbed the _red_ rays, which,
when mixed with the _yellow_ on one side, constituted _orange_, and the
_blue_ rays, which, when mixed with the _yellow_ on the other side,
constituted _green_, so that the insulation of the _yellow_ rays thus
effected, and the disappearance of the _orange_, and of the greater
part of the _green_ light, proves beyond a doubt that the _orange_
and _green_ colours in the spectrum are compound colours, the former
consisting of _red_ and _yellow_ rays, and the latter of _yellow_ and
_blue_ rays _of the very same refrangibility_. If we compare the two
red spaces of the spectrum seen through the blue glass with the red
space seen without the blue glass, it will be obvious that the red
has experienced such an alteration in its tint by the action of the
blue glass, as would be effected by the absorption of a small portion
of yellow rays; and hence we conclude, that the red of the spectrum
contains a slight tinge of yellow, and that the yellow space extends
over more than one-half of the spectrum, including the _red_, _orange_,
_yellow_, _green_, and _blue_ spaces.
I have found also that red light exists in the yellow space, and it
is certain that in the violet space red light exists in a state of
combination with the blue rays. From these and other facts which it
would be out of place here to explain, I conclude that the prismatic
spectrum consists of three different spectra, viz. red, yellow, and
blue, all having the same length, and all overlapping each other. Hence
red, yellow, and blue rays of the very same refrangibility coexist at
every point of the spectrum; but the colour at any one point will be
that of the predominant ray, and will depend upon the relative distance
of the point from the maximum ordinate of the curve which represents
the intensity of the light of each of the three spectra.
[Illustration: _Fig. 7._]
This structure of the spectrum, which harmonizes with the old
hypothesis of three simple colours, will be understood from the annexed
diagram, where MN is the spectrum of seven colours, all compounded of
the three simple ones, _red_, _yellow_, and _blue_. The ordinates of
the curves R, Y, and B will express the intensities of each colour
at different points of the spectrum. At the red extremity M of the
spectrum, the pure _red_ is scarcely altered by the very slight
intermixture of yellow and blue. Farther on in the red space, the
_yellow_ begins to make the red incline to scarlet. It then exists
in sufficient quantity to form orange, and, as the red declines, the
yellow predominates over the feeble portion of red and blue which are
mixed with it. As the yellow decreases in intensity, the increasing
blue forms with it a good green, and the blue rising to its maximum
speedily overpowers the small portion of yellow and red. When the blue
becomes very faint, the red exhibits its influence in converting it
into violet, and the yellow ceases to exercise a marked influence on
the tint. The influence of the red over the blue space is scarcely
perceptible, on account of the great intensity of the blue light; but
we may easily conceive it to reappear and form the violet light, not
only from the rapid decline of the blue light, but from the greater
influence of the red rays upon the retina.
These views may, perhaps, be more clearly understood by supposing that
a certain portion of white light is actually formed at every point of
the spectrum by the union of the requisite number of the three coloured
rays that exist at any point. The white light thus formed will add to
the brilliancy without affecting the tint of the predominant colour.
In the violet space we may conceive the small portion of yellow which
exists there to form white light with a part of the blue and a part
of the red, so that the resulting tint will be violet, composed of
the blue and the small remaining portion of red, mixed with the white
light. This white light will possess the remarkable property of not
being susceptible of decomposition by the analysis of the prism,
as it is composed of red, yellow, and blue rays of the very same
refrangibility. The insulation of this white light by the absorption
of the predominant colours I have effected in the green, yellow, and
red spaces, and by the use of new absorbing media we may yet hope to
exhibit it in some of the other colours, particularly in the brightest
part of the blue space, where an obvious approximation to it takes
place.
Among the most important modern discoveries respecting the spectrum
we must enumerate that of fixed dark and coloured lines, which we owe
to the sagacity of Dr. Wollaston and M. Fraunhofer. Two or three of
these lines were discovered by Dr. Wollaston, but nearly 600 have been
detected by means of the fine prisms and the magnificent apparatus
of the Bavarian optician. These lines are parallel to one another,
and perpendicular to the length of the spectrum. The largest occupy a
space from 5″ to 10″ in breadth. Sometimes they occur in well-defined
lines, and at other times in groups; and in all spectra formed from
solar light, they preserve the same order and intensity, and the same
relative position to the coloured spaces, whatever be the nature of the
prism by which they are produced. Hence these lines are fixed points,
by which the relative dispersive powers of different media may be
ascertained with a degree of accuracy hitherto unknown in this branch
of science. In the light of the fixed stars, and in that of artificial
flames, a different system of lines is produced, and this system
remains unaltered, whatever be the nature of the prism by which the
spectrum is formed.
The most important fixed lines in the spectrum formed by light emitted
from the sun, whether it is reflected from the sky, the clouds, or
the moon, may be easily seen by looking at a narrow slit in the
window-shutter of a dark room, through a hollow prism formed of
plates of parallel glass, and filled with any fluid of a considerable
dispersive power. The slit should not greatly exceed the twentieth of
an inch, and the eye should look through the thinnest edge of the prism
where there is the least thickness of fluid. These lines I have found
to be the boundaries of spaces within which the rays have particular
affinities for particular bodies.