ACHROMATIC
, in Optics, without colour; a term which, it seems, was first used by M. de la Lande, in his astronomy, to denote telescopes of a new invention, contrived to remedy aberrations and colours. See Aberration and Telescope.
Achromatic Telescope, a singular species of refracting telescope, said to be invented by the late Mr. John Dollond, optician to the king, and since improved by his son Mr. Peter Dollond, and others.
Every ray of light passing obliquely from a rarer into a denser medium, changes its direction towards the perpendicular; and every ray passing obliquely from a denser into a rarer medium, changes its direction from the perpendieular. This bending of the ray, caused by the change of its direction, is called its refraction; and the quality of light which subjects it to this refraction, is called its refrangibility. Every ray of light, before it is refracted, is white, though it consists of a number of component rays, each of which is of a different colour. As soon as it is refracted, it is separated into its component rays, which, from that time, proceed diverging from each other, like rays from a centre: and this divergency is caused by the different refrangibility of the component rays, in such sort, that the more the original or component ray is refracted, the more will the compound rays diverge when the light is refracted by one given medium only.
From hence it has been concluded, that any two different mediums that can be made to produce equal refractions, will necessarily produce equal divergencies: whence it should also follow, that equal and contrary refractions should not only destroy each other, but that the divergency of the colours caused by one refraction, should be corrected by the other; and that to produce refraction that would not be affected by the different refrangibility of light, is impossible.
But Mr. Dollond has proved, by many experiments, that these conclusions are not well founded; from which experiments it appeared, that a ray of light, after equal and contrary refractions, was still spread into component rays differently coloured: in other words, that two different mediums may cause equal refraction, but different divergency; and equal divergency, with different refraction. It follows therefore that refraction may be produced, which is not affected by the different refrangibility of light. In other words, that, if the mediums be different, different refractions may be produced, though at the same time the divergency caused by one refraction shall be exactly counteracted by the divergency caused by the other; and so an object may be seen through mediums which, together, cause the rays to converge, without appearing of different colours.
This is the foundation of Mr. Dollond's improvement of refracting telescopes. By subsequent experiments he found, that different sorts of glass differed greatly in their refractive qualities, with respect to the divergency of colours. He found that crown glass causes the least diver- gency, and white flint the most, when they are wrought into forms that produce equal refractions. He ground a piece of white flint glass into a wedge, whose angle was about 25 degrees; and a piece of crown glass to another, whose angle was about 29 degrees; and these he found refracted nearly alike, but that their divergency of colours was very different.
He then ground several other pieces of crown glass to wedges of different angles, till he got one that was equal, in the divergency it produced, to that of a wedge of flint glass of 25 degrees; so that when they were put together, in such a manner as to refract in contrary directions, the refracted light was perfectly free from colour. Then measuring the fractions of each wedge, he found that that of the white flint glass, was to that of the crown glass, nearly as two to three. And hence any two wedges, made of these two substances, and in this proportion, would, when applied together so as to refract in contrary directions, refract the light without any effect ariling from the different refrangibility of the component rays.
Therefore, to make two spherical glasses that refract the light in contrary directions, one must be concave, and the other convex; and as the rays, after passing through both, must meet in a focus, the excess of the refraction must be in the convex one: and as the convex is to refract most, it appears from the experiment that it must be made of crown glass; and as the concave is to refract least, it must be made of white flint.
And farther, as the refractions of spherical glasses are in an inverse ratio of their focal distances, it follows that the focal distances of the two glasses should be in the ratio of the refractions of the wedges; for, being thus proportioned, every ray of light that passes through this combined glass, at whatever distance from its axis, will constantly be refracted by the difference between two contrary refractions, in the proportion required; and therefore the effect of the different refrangibility of light will be prevented.
The removal of this impediment, however, produced another: for the two glasses, which were thus combined, being segments of very deep spheres, the aberrations from the spherical surfaces became so considerable, as greatly to disturb the distinctness of the image. Yet considering that the surfaces of spherical glasses admit of great variations, though the focal distance be limited, and that by these variations their aberration might be made more or less at pleasure; Mr. Dollond plainly saw that it was possible to make the aberrating of any two glasses equal; and that, as in this case the refractions of the two glasses were contrary to each other, and their aberrations being equal, these would destroy each other.
Thus he obtained a persect theory of making object glasses, to the apertures of which he could hardly perceive any limits: for if the practice could come up to the theory, they must admit of apertures of great extent, and consequently bear great magnifying powers.
The difficulties of the practice are, however, still very considerable. For first, the focal distances, as well as the particular surfaces, must be proportioned with the utmost accuracy to the densities and refracting powers of the glasses, which vary even in the same sort of glass, when made at different times. Secondly, there are four surfaces to be wrought persectly spherical. | However, Mr. Dollond could construct refracting telescopes upon these principles, with such apertures and magnifying powers, under limited lengths, as greatly exceed any that were produced before, in forming the images of objects bright, distinct, and uninfected with colours about the edges, through the whole extent of a very large field or compass of view; of which he has given abundant and undeniable testimony. See TELESCOPE.
There has lately appeared in the Gentleman's Magazine (1790, pa. 890) a paper on the refracting telescope, by an author who signs Veritus, in which the invention is ascribed to another person, not heretofore mentioned; in these words: “As the invention has been claimed by M. Euler, M. Klingenstierna, and some other foreigners, we ought, for the honour of England, to assert our right, and give the merit of the discovery to whom it is due; and therefore, without farther preface, I shall observe, that the inventor was Chester More Hall, Esq. of More-hall, in Essex, who, about 1729, as appears by his papers, considering the different humours of the eye, imagined they were placed so as to correct the different refrangibility of light. He then conceived, that if he could find substances having such properties as he supposed these humours might possess, he should be enabled to construct an object glass that would shew objects colourless. After many experiments he had the good fortune to find those properties in two different sorts of glass, and making them disperse the rays of light in different directions, he succeeded. About 1733 he completed several achromatic object glasses (though he did not give them this name), that bore an aperture of more than 2 1/2 inches, though the focal length did not exceed 20 inches; one of which is now in the possession of the Rev. Mr. Smith, of Charlotte Street, Rathbone Place.
This glass has been examined by several gentlemen of eminence and scientific abilities, and found to possess the properties of the present achromatic glasses.
Mr. Hall used to employ the working opticians to grind his lenses; at the same time he finished them with the radii of the surfaces, not only to correct the different refrangibility of rays, but also the aberration arising from the spherical figure of the lenses. Old Mr. Bass, who at that time lived in Bridewell precinct, was one of these working opticians, from whom Mr. Hall's invention seems to have been obtained.
In the trial at Westminster hall about the patent for making achromatio telescopes, Mr. Hall was allowed to be the inventor; but Lord Mansfield observed, that “It was not the person that locked up his invention in his scrutoire that ought to profit by a patent for such an invention, but he who brought it forth for the benefit of the public.” This, perhaps, might be said with some degree of justice, as Mr. Hall was a gentleman of property, and did not look to any pecuniary advantage from his discovery; and, consequently, it is very probable that he might not have an intention to make it generally known at that time.
That Mr. Ayscough, optician on Ludgate Hill, was in possession of one of Mr. Hall's achromatic telescopes in 1754, is a fact which at this time will not be disputed.”
