DISCOVERY OF ACHROMATISM.
THE discovery that the laws of refractive dispersion of different
substances were such as to allow of combinations which neutralised
the dispersion without neutralizing the refraction, is one which has
hitherto been of more value to art than to science. The property has
no definite bearing, which has yet been satisfactorily explained,
upon the _theory_ of light; but it is of the greatest importance in
its application to the construction of telescopes; and it excited
the more notice, in consequence of the prejudices and difficulties
which for a time retarded the discovery.
Newton conceived that he had proved by experiment,[31\9] that light
{67} is white after refraction, when the emergent rays are parallel
to the incident, and in no other case. If this were so, the
production of colorless images by refracting media would be
impossible; and such, in deference to Newton's great authority, was
for some time the general persuasion. Euler[32\9] observed, that a
combination of lenses which does not color the image must be
possible, since we have an example of such a combination in the
human eye; and he investigated mathematically the conditions
requisite for such a result. Klingenstierna,[33\9] a Swedish
mathematician, also showed that Newton's rule could not be
universally true. Finally, John Dollond,[34\9] in 1757, repeated
Newton's experiment, and obtained an opposite result. He found that
when an object was seen through two prisms, one of glass and one of
water, of such angles that it did not appear displaced by
refraction, it was colored. Hence it followed that, without being
colored, the rays might be made to undergo refraction; and that
thus, substituting lenses for prisms, a combination might be formed,
which should produce an image without coloring it, and make the
construction of an _achromatic_ telescope possible.
[Note 31\9: _Opticks_, B. i. p. ii. Prop. 3.]
[Note 32\9: _Ac. Berlin._ 1747.]
[Note 33\9: _Swedish Trans._ 1754.]
[Note 34\9: _Phil. Trans._ 1758.]
Euler at first hesitated to confide in Dollond's experiments; but he
was assured of their correctness by Clairaut, who had throughout
paid great attention to the subject; and those two great
mathematicians, as well as D'Alembert, proceeded to investigate
mathematical formulæ which might be useful in the application of the
discovery. The remainder of the deductions, which were founded upon
the laws of dispersion of various refractive substances, belongs
rather to the history of art than of science. Dollond used at first,
for his achromatic object-glass, a lens of crown-glass, and one of
flint-glass. He afterwards employed two lenses of the former
substance, including between them one of the latter, adjusting the
curvatures of his lenses in such a way as to correct the
imperfections arising from the spherical form of the glasses, as
well as the fault of color. Afterwards, Blair used fluid media along
with glass lenses, in order to produce improved object-glasses. This
has more recently been done in another form by Mr. Barlow. The
inductive laws of refraction being established, their results have
been deduced by various mathematicians, as Sir J. Herschel and
Professor Airy among ourselves, who have simplified and extended the
investigation of the formulæ which determine the best combination of
lenses in the object-glasses and eye-glasses of {68} telescopes,
both with reference to spherical and to _chromatic_ aberrations.
According to Dollond's discovery, the colored spectra produced by
prisms of two substances, as flint-glass and crown-glass, would be
of the same length when the refraction was different. But a question
then occurred: When the whole distance from the red to the violet in
one spectrum was the same as the whole distance in the other, were
the intermediate colors, yellow, green, &c., in corresponding places
in the two? This point also could not be determined any otherwise
than by experiment. It appeared that such a correspondence did not
exist; and, therefore, when the extreme colors were corrected by
combinations of the different media, there still remained an
uncorrected residue of color arising from the rest of the spectrum.
This defect was a consequence of the property, that the spectra
belonging to different media were not divided in the _same ratio_ by
the same colors, and was hence termed the _irrationality_ of the
spectrum. By using three prisms, or three lenses, three colors may
be made to coincide instead of two, and the effects of this
irrationality greatly diminished.
For the reasons already mentioned, we do not pursue this subject
further,[35\9] but turn to those optical facts which finally led to
a great and comprehensive theory.
[Note 35\9: The discovery of the _fixed lines_ in the spectrum, by
Wollaston and Fraunhofer, has more recently supplied the means of
determining, with extreme accuracy, the corresponding portions of
the spectrum in different refracting substances.]
[2nd Ed.] [Mr. Chester More Hall, of More Hall, in Essex, is said to
have been led by the study of the human eye, which he conceived to
be achromatic, to construct achromatic telescopes as early as 1729.
Mr. Hall, however, kept his invention a secret. David Gregory, in
his _Catoptrics_ (1713), had suggested that it would perhaps be an
improvement of telescopes, if, in imitation of the human eye, the
object-glass were composed of different media. _Encyc. Brit._ art.
_Optics_.
It is said that Clairaut first discovered the irrationality of the
colored spaces in the spectrum. In consequence of this
irrationality, it follows that when two refracting media are so
combined as to correct each other's extreme dispersion, (the
separation of the red and violet rays,) this first step of
correction still leaves a residue of {69} coloration arising from
the unequal dispersion of the intermediate rays (the green, &c.).
These _outstanding_ colors, as they were termed by Professor
Robison, form the residual, or _secondary_ spectrum.
Dr. Blair, by very ingenious devices, succeeded in producing an
object-glass, corrected by a fluid lens, in which this aberration of
color was completely corrected, and which performed wonderfully well.
The dispersion produced by a prism may be corrected by another prism
of the _same substance_ and of a different angle. In this case also
there is an irrationality in the colored spaces, which prevents the
correction of color from being complete; and hence, a new residuary
spectrum, which has been called the _tertiary_ spectrum, by Sir
David Brewster, who first noticed it.
I have omitted, in the notice of discoveries respecting the
spectrum, many remarkable trains of experimental research, and
especially the investigations respecting the power of various media
to absorb the light of different parts of the spectrum, prosecuted
by Sir David Brewster with extraordinary skill and sagacity. The
observations are referred to in chapter iii. Sir John Herschel,
Prof. Miller, Mr. Daniel, Dr. Faraday, and Mr. Talbot, have also
contributed to this part of our knowledge.]