ACHROMATIC

, pa. 25, col. 2, l. 1^.4, for fractions, read refractions.

Pa. 26, col. 1, l. 12, for Veritus, read Veritas.

After l. 9, add, Since this article was printed, I observe, in the 3d volume of the Edinburgh Philosophical Transactions, an account of a curious set of experiments, on the unequal refrangibility of light, with observations on Achromatic telescopes, by Dr. Robert Blair. This ingenious gentleman sets out with observing, “If the theory of the Achromatic telescope is so complete as it has been represented, may it not reasonably be demanded, whence it proceeds, that Hugenius and others could execute telescopes with single object glasses 8 inches and upwards in diameter, while a compound object glass of half these dimensions, is hardly to be met with? or how it can arise from any defect in the execution, that reflectors can be made so much shorter than Achromatic refractors of equal apertures, when it is well known that the latter are much less affected by any imperfections in the execution of the lenses composing the object glass, than reflectors are by equal defects in the figure of the great speculum?—The general answer made by artists to enquiries of this kind, is, that the fault lies in the imperfection of glass, and particularly in that kind of glass of which the concave lens of the compound object glass is formed, called flint glass.— It was in order to satisfy myself concerning the reality of this difficulty, and to attempt to remove it, that I engaged in the following course of experiments.”

Dr. Blair describes the apparatus and manner of making the experiments. He employed various prisins of different kinds of glass; also lenses of glass, and of a great variety of fluid mediums, having different degrees of refraction. Having detailed the whole at considerable length, for which a reference must be made to the work itself, and it is very deserving of attentive perusal, he concludes with the following recapitulation of the contents and scope of the whole discourse.

“The unequal refrangibility of light, as discovered and fully explained by Sir Isaac Newton, so far stands its ground uncontroverted, that when the refraction is made in the confine of any medium whatever, and a vacuum, the rays of different colours are unequally refracted, the red-making rays being the least refrangible, and the violet-making rays the most refrangible.

“The discovery of what has been called a different dispersive power in different refractive mediums, proves those theorems of Sir Isaac Newton not to be universal, in which he concludes that the difference of refraction of the most and least refrangible rays, is always in a given proportion to the refraction of the mean refrangible ray. There can be no doubt that this position is true with respect to the mediums on which he made his experiments; but there are many exceptions to it.

“For the experiments of Mr. Dollond prove, that the difference of refraction between the red and violet rays, in proportion to the refraction of the whole pencil, is greater in some kinds of glass than in water, and greater in flint-glass than in crown-glass.

“The first set of experiments above recited, prove, that the quality of dispersing the rays in a greater degree than crown-glass, is not confined to a few mediums, but is possessed by a great variety of fluids, and by some of these in a most extraordinary degree. Solutions of metals, essential oils, and mineral acids, with the exception of the vitriolic, are most remarkable in this respect.

“Some consequences of the combinations of mediums of different dispersive powers, which have not been sufficiently attended to, are then explained. Although the greater refrangibility of the violet rays than of the red rays, when light passes from any medium whatever into a vacuum, may be considered as a law of nature, yet in the passage of light from one medium into another, it depends entirely on the qualities of the mediums, which of these rays shall be the most refrangible, or whether there shall be any difference in their refrangibility. |

“The application of the demonstrations of Hugenius to the correction of the aberration from the spherical figures of lenses, whether solid or fluid, is then taken notice of, as being the next step towards perfecting the theory of telescopes.

“Next it appears from trials made with object-glasses of very large apertures, in which both aberrations are corrected as far as the principles will admit, that the correction of colour which is obtained by the common combination of two mediums which differ in dispersive power, is not complete. The homogeneal green rays emerge most refracted, next to these the united blue and yellow, then the indigo and orange united, and lastly the united violet and red, which are least refracted.

“If this production of colour were constant, and the length of the secondary spectrum were the same in all combinations of mediums when the whole refraction of the pencil is equal, the perfect correction of the aberration from difference of refrangibility would be impossible, and would remain an insurmountable obstacle to the improvement of dioptrical instruments.

“The object of the next experiment is, therefore, to search, whether nature affords mediums which differ in the degree in which they disperse the rays composing the prismatic spectrum, and at the same time separate the several orders of rays in the same proportion. For if such could be found, the above-mentioned secondary spectrum would vanish, and the aberration from difference of refrangibility might be removed. The result of this investigation was unsuccessful with respect to its principal object. In every combination that was tried, the same kind of uncorrected colour was observed, and it was thence concluded, that there was no direct method of removing the aberration.

“But it appeared in the course of the experiments, that the breadth of the secondary spectrum was less in some combinations than in others, and thence an indirect way opened, leading to the correction sought after; namely by forming a compound concave lens of the materials which produce most colour, and combining it with a compound convex lens formed of the materials which produce least colour; and it was observed in what manner this might be effected by means of three mediums, though apparently four are required.

“In searching for mediums best adapted for the above purpose, a very singular and important quality was detected in the muriatic acid. In all the dispersive mediums hitherto examined, the green rays, which are the mean refrangible in crown-glass, were found among the less refrangible, and thence occasion the uncorrected colour which has been described. In the muriatic acid, on the contrary, these same rays make a part of the more refrangible; and in consequence of this, the order of the colours in the secondary spectrum, formed by a combination of crown glass with this fluid, is inverted, the homogeneal green being now the least refrangible, and the united red and violet the most refrangible.

“This remarkable quality found in the marine acid led to complete success in removing the great defect of optical instruments, that dissipation or aberration of the rays, arising from their unequal refrangibility, which has rendered it impossible hitherto to converge all of them to one point either by single or opposite refractions. A fluid in which the particles of marine acid and metal- line particles hold a due proportion, at the same time that it separates the extreme rays of the spectrum much more than crown-glass, refracts all the orders of rays exactly in the same proportion as the glass does; and hence rays of all colours, made to diverge by the refraction of the glass, may either be rendered parallel by a subsequent refraction made in the confine of the glass and this fluid, or by weakening the refractive density of the fluid, the refraction which takes place in the confine of it and glass, may be rendered as regular as reflexion, while the errors arising from unavoidable imperfections of workmanship, are far less hurtful than in reflexion, and the quantity of light transmitted by equal apertures of the telescopes much greater.

“Such are the advantages which the theory presents. In reducing this theory to practice, difficulties must be expected in the first attempts. Many of these it was necessary to surmount before the experiments could be completed. For the delicacy of the observations is such as to require a considerable degree of perfection in the execution of the object-glasses, in order to admit of the phenomena being rendered more apparent by means of high magnifying powers. Great pains seem to have been taken by mathematicians to little purpose, in calculating the radii of the spheres requisite for Achromatic telescopes, from their not considering that the object-glass itself is a much nicer test of the optical properties of refracting mediums than the gross experiments made by prisms, and that the results of their demonstrations cannot exceed the accuracy of the data, however much they may fall short of it.

“I shall conclude this paper, which has now greatly exceeded its intended bounds, by enumerating the several cases of unequal refrangibility of light, that their varieties may at once be clearly apprehended.

“In the refraction which takes place in the confine of every known medium and a vacuum, rays of different colours are unequally refrangible, and the red-making rays are least refrangible, and the violet-making rays are most refrangible.

“This difference of refrangibility of the red and violet rays is not the same in all mediums. Those mediums in which the difference is greatest, and which, by consequence, separate or disperse the rays of different colours most, have been distinguished by the term dispersive, and those mediums which separate the rays least have been called indispersive. Dispersive mediums differ from indispersive, and still more from each other, in another very essential circumstance.

“It appears from the experiments which have been made on indispersive mediums, that the mean refrangible light is always the same, and of a green colour.

“Now, in by far the largest class of dispersive mediums, including flint glass, metallic solutions, essential oils, the green light is not the mean refrangible order, but forms one of the less refrangible orders of light, being found in the prismatic spectrum nearer to the deep red than the extreme violet.

“In another class of dispersive mediums, which includes the muriatic and nitrous acids, this same green light becomes one of the more refrangible orders, being now found nearer to the extreme violet than the deep red. |

“These are the varieties in the refrangibility of light, when the refraction takes place in the confine of a vacuum; and the phenomena will scarce differ sensibly in refractions made in the confine of dense mediums and air.

“But when light passes from one dense medium into another, the cases of unequal refrangibility are more complicated.

“In refractions made in the confine of mediums which differ only in strength, not in quality, as in the confine of water and crown-glass, or in the confine of the different kinds of dispersive fluids more or less diluted, the difference of refrangibility will be the same as above stated in the confine of dense mediums and air, only the whole refiaction will be less.

“In the confine of an indispersive medium, and a rarer medium belonging to either class of the dispersive, the red and violet rays may be rendered equally refrangible. If the dispersive power of the rare medium be then increased, the violet rays will become the least refrangible, and the red rays the most refrangible. If the mean refractive density of the two mediums be rendered equal, the red and violet rays will be refracted in opposite directions, the one towards, the other from the perpendicular.

“Thus it happens to the red and violet rays, whichsoever class of dispersive mediums be employed. But the refrangibility of the intermediate orders of rays, and especially of the green rays, will be different when the class of dispersive mediums is changed.

“Thus, in the first case, where the red and violet rays are rendered equally refrangible, the green rays will emerge most refrangible if the first class of dispersive mediums is used, and least refrangible if the second class is used. And in the other two cases, where the violet becomes least refrangible, and the red most refrangible, and where these two kinds of rays are refracted in opposite directions, the green rays will join the red if the first class of dispersive mediums be employed, and will arrange themselves with the violet if the second class be made use of.

“Only one case more of unequal refrangibility remains to be stated; and that is, when light is refracted in the confine of mediums belonging to the two different classes of dispersive fluids. In its transition, for example, from an essential oil, or a metallic solution, into the muriatic acid, the refractive density of these fluids may be so adjusted, that the red and violet rays shall suffer no refraction in passing from the one into the other, how oblique soever their incidence be. But the green rays will then suffer a considerable refraction, and this refraction will be from the perpendicular, when light passes from the muriatic acid into the essential oil, and towards the perpendicular, when it passes from the essential oil into the muriatic acid. The other orders of rays will suffer similar refractions, which will be greatest in those adjoining the green, and will diminish as they approach the deep red on the one hand, and the extreme violet on the other, where the refraction ceases entirely.

“The manner of the production of these effects, by the attraction of the several mediums, may be thus explained. We shall suppose the attractive forces, which produce the refractions of the red, green and violet light, to be represented by the numbers, 8, 12, and 16, in glass; 6, 9, 14, in the metallic solution; 6, 11, 14, in the muriatic acid; and 6, 10, 14, in a mixture of these two fluids. The excess of attraction of glass for the red and violet light is equal to 2, whichsoever of the three fluids be employed. The refraction of these two orders of rays will therefore be the same in all the three cases. But the excess of attraction for the green light is equal to 3, when the metallic solution is used, and therefore the green light will be more refracted than the red and violet, in this case. When the muriatic acid is used, the excess of attraction of glass for the green light is only 1, and therefore the green light will now be less refracted than the red and violet.

“We shall next suppose the metallic solution and the acid to adjoin each other. The attractions of both these mediums, for the red light being 6, and for the violet light 14, these two orders of rays will suffer no refraction in the confine of the two fluids, the difference of their attractions being equal to nothing.

“But the attractive force of the metallic solution for the green ray being only 9, and that of the muriatic acid for the same ray being 11, the green light will be attracted towards the muriatic acid with the force 2; and therefore the difference between the refraction of the green light and the unrefracted red and violet light, which takes place in the confine of these fluids, will greatly exceed the difference of refraction of the green light, and equally refracted red and violet light, which is produced in the confine of glass and either of the fluids.

“Lastly, in a mixture of the two kinds of fluids, the attraction for the red, green and violet rays, being 6, 10 and 14, and that of the glass, 8, 12 and 16, the excess of the attraction of the glass for the green rays, is the same which it is for the red and violet rays. These three orders of rays will therefore suffer an equal refraction, being each of them attracted towards the glass with the force 2; and when this is the case, it appears, from the observations, that the indefinite variety of rays of intermediate colours and shades of colours, which altogether compose solar light, will also be regularly bent from their rectilinear course, constituting what has been termed a planatic refraction.”

In short, Dr. Blair says, that he “uses more transparent mediums than the common ones; avoids or greatly diminishes the reflections at the surfaces of the mediums; applies fluid mediums more homogeneous than thick flint or crown glass, which at the same time disperse the different coloured rays of light in the same proportion, by which means an image is produced perfectly Achromatic, which is but imperfectly so in Dollond's object glasses made of flint and crown glass combined.