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 undergone64 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 of65 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.

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,66 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,67 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. by68 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,69 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 the70 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,71 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;72 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.

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 ceases73 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 are74 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.