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CHAP. XXVI.

DESCRIPTION OF SOME NEW OPTICAL INSTRUMENTS,

WITH EXPERIMENTS ON

LIGHT AND COLOURS.

Although by Nature's liberal bounty bless'd,
The fire of Genius glow within the breast,
Collateral studies still must feed the flame
That, clearly burning, brightens into fame.
Da Vinci thus the light of science sought,
And Art, reviving, kindled as he wrought;
Thus Buonaroti rear'd his lofty name,
And great Urbino brighten'd into fame;
Rubens array'd in learning's lustre shone,
And triumph'd on his allegoric throne;

Thus, graced with all that liberal studies yield
To form the powers of genius for the field,
Accomplish'd Reynolds claims the Muse's praise.
SHEE, ELEMENTS OF ART, p. 248.

THE principal of these instruments are the CHROMASCOPE and METROCHROME, so named from xpõμa, colour; σxoñéw, to view; and perpéw, to measure. They are optical instruments appropriate to colours, of which the author published some account several years ago; and, as they have since excited some interest, are intimately connected with the subject of this work, and may

In an essay, entitled Esthetics, or the Analogy of the Sensible Sciences. Pamphleteer, No. XXXIII., and previously in his Chromatics, or Essay on the Analogy and Harmony of Colours.

conduce to farther instructive experience or liberal amusement, he has deemed a concise description of their construction, application, and results to be a proper appendage to this work.

His aim in these contrivances has been, by augmenting the power of the prism in the one, and adapting it to a mode of measurement in the other, to determine upon what degree of evidence the dogma of the infrangibility of Newton's homogeneal colours rested,* to illustrate the preceding doctrine of the specific powers, the relations, and harmony of colours, delivered more at large in a former work, † and to establish a standard of colours.

The common triangular glass prism has been consecrated to science by the genius of Newton as the instrument which, while it exhibits the beauties and wonders of light and colours, unfolds also the mystery of their union and separation; it has accordingly held a principal place among the instruments of the natural philosopher, unvaried and unimproved to this day.

It is geometrically evident, notwithstanding, that as the figure of the common prism is generated by the rectilinear motion of a triangle, so it is capable of infinite variation, according to figures generated by a circular, angular, or compound motion of a triangle. Accordingly, by the motion of a triangle A B C, fig. 1, pl. 11., round one of its sides A B, as an axis, is formed, as it were a circular prism, which, from its uniting the properties and figure of a lens with those of the prism, may be called a LEnsic, or LENTICULAR PRISM, or double convex PRISMATIC, or PRISMIC LENS A B C D, and fig. 4. A BIK.

Again, by a like circular motion of a triangle EF G, pl. 11. fig. 2, upon the angle F, which corresponds to the angle of refraction in a common prism, will be generated a similar double concave lensic prism, opposed to the above, E F G H, and fig. 4, U KIT.

The figures above mentioned are sufficient for the present purpose, yet it is evident that they are extremes, between which lie an indefinite series of

* Optics, Exp. v. Theor. II. Prop. 11. p. 106, &c. See Exp. XII. XIII. following.

+ Chromatics, or an Essay on the Analogy and Harmony of Colours.

‡ Nothing is either above or beneath the attention of the true philosopher. The mind of Newton was too great to despise even the toys of children; soap-bubbles blown from a tobaccopipe, and the prism, long before known as a toy under the appellation of Fool's Paradise, became in his hand simple, yet mighty instruments of science.

intermediate figures, and that there are innumerable others, both conical and annular, generable upon the boundless variety of figure and motion. It is obvious also that prismatic specula may be constructed upon the same principle, which will afford by reflection optical effects analogous to those of these lensic prisms by refraction. We thus produce new secondary optical powers, the primary species of which are the lens, the speculum, and the prism, adapted to the three general habits of light by which it is transmitted, reflected, or refracted; whence arise the various powers of artificial vision, and the wonderful effects of all optical instruments. It is evident then that lensic prisms may be constructed according to all the various figures of lenses.

Of lenses there are three simple or primary kinds,—the plane, the convex, and the concave; of these forms are compounded three secondary species of lenses; the plano-conver, the plano-concave, and the concavo-convex, or meniscus; and these comprehend all the generic forms of lenses enumerated from the earliest times in books of optics. Such also are the variety of lensic prisms.

To facilitate the use of these lensic prisms, in a variety of experiments, the instrument called a CHROMASCOPE has been constructed, of which the following is a brief description.

A, fig. 3. pl. 11., represents the brass tube of the chromascope, nearly two inches in diameter, and about five inches long; at one end of which is fixed, by a screw-collar, the plano-convex, or other lensic prism B; and at the other end is a small sliding tube, about two inches long and half an inch in diameter, for holding an eye-glass of seven-inch focus, or occasionally small lensic prisms; and, as a guide for the eye to the centre of the principal tube, it is fitted with a screw-cap having a small perforation at C.

The whole is held by, and slides in, the short tube, or collar D, connected with a supporting tripod having a universal joint, by which the chromascope may be turned from the horizontal position to the vertical, dotted at E, or otherwise elevated at any angle, or in any direction, for viewing objects, G, on a table, or on a portable screen or tablet F, &c.

This description of one form of the chromascope will be sufficient for a clear comprehension of the following experiments.

EXPERIMENT I.

In the centre of a white card, six inches square at least, form a black spot th of an inch diameter. Place the card upon the table G, fig. 3, pl. II., in sunshine, or a clear light near a window, and so adjust the chromascope over it, in a vertical position, that the spot may be close to, and concentrical with the lensic prism; then (having removed the lens of the eye-piece, which is unnecessary in this experiment) gradually sliding the chromascope upward, looking at the same time through the tube, the spot will appear to expand and become refracted into a beautiful annular spectrum, or aureola of the three primary colours, resembling a rainbow, as represented pl. 1. fig. 1.

If now, under these circumstances, a concave lensic prism, pl. 11. fig. 2, of the same refractive power of the convex prism of the chromascope be interposed between it and the object, the aureola will be, by a counter refraction, reduced to a black spot at the centre.

REMARKS.-It would be difficult to account satisfactorily for the production of colours in the above experiment by the analysis of simple light, since the coloured spectrum would vanish if the spot were removed. It is to be presumed, therefore, that the principle of shade from the spot concurs with the principle of light from the ground, by the medium of the lensic prism, in producing the circular iris. This is apparent also from the next experiment.

This concurrence of shade with light is demonstrable in all similar effects of prisms and prismic lenses in which coloured spectra are produced, although no account has been taken thereof, such phenomena having been attributed to the sole effects of light. This concurrence of shade and light is remarkable in the experiment of Newton, Optics, b. 1. obs. 6, on the inflections of light and their colours, and affords easy explanation of all the experiments and observations contained in the third book of his Optics, and particularly obs. 6.

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