with the lenticular prism: then gradually sliding the tube upward, looking at the same time through it, the spot will appear to expand and be refracted into a beautiful, annular, colored spectrum or aureola,

If now the double concave lenticular prism [Fig. 2.] be interposed between the chromascope and the spot, the aureola will be, by a counter refraction, reduced to the spot at its centre.

REMARKS. It would be difficult to account satisfactorily for the production of colors in the above experiment, by the analysis of light alone, since the colored spectrum would vanish if the black spot were removed. It is to be presumed, therefore, that the principle of shade in the spot, concurs with the principle of light in the ground in producing the circular iris.

If the preceding experiment be performed with a white spot upon a black ground, in place of the black spot upon the white ground, a similar spectrum will be produced, in which the order of the colors will be inverted. It is not necessary that the objects and grounds opposed be black and white, to produce a spectrum; it is sufficient that they be lighter and darker with reference to each other; nor is it necessary that they be not colored, since a blue, red or yellow spot, upon a ground lighter or darker than itself, yields a colored spectrum similar to the above, in which, notwithstanding, the particular color of the spot itself predominates, each of the primary colors appears in remarkable coincidence with the consonance of the primary triad in every musical sound demonstrated by Mercennus and Dr. Wallis.

REMARKS. Various doctrines have prevailed with respect to the number of the primary colors, from one to seven; but the last of these opinions, sanctioned by the genius of Newton, and the apparent cogency of his attempt to demonstrate the geometrical analogy of these seven colors with the diatonic octave of modern music, has been most generally received. If, however, the coincidence of the three colors, blue, red and yellow, with the consonance of the primary musical triad C, E, G, be the true foundation of such analogy, and if it be incontrovertibly demonstrable that all other colors may be composed of these three, and that only is primitive and elementary which cannot be composed, as is the case of these three colors, then are they the only true primary

Governor Pownall has advanced an hypothesis of one only primary color, red, declining through orange into yellow, blue being only a deprivation of light, &c. Phil. Mag. x. p. 111.

2 See § 10.

colors, and as such the cultivated eye of the artist has regarded them.

A distinguished member of the Royal Society has, however, controverted both these opinions, and pronounced the number of the primary colors to be four; because on looking through a prism at a beam of light 10 or 12 feet distant, he saw distinctly that number of colors. Had he chanced to have viewed the light within an inch or two of its source, he would have discovered that his fourth primitive arose from the crossing of the rays of blue and yellow.

Had Newton, too, examined his spectrum near its egress from the prism, he would have perceived that his green, orange, violet and indigo primitives arose from similar crossings of the blue, red and yellow rays.

A professor of Frankfort on the Oder, has published a work, to prove "that light consists not of seven but of three primary colors, red, green and violet; by mingling prismatic streaks of red and green a bright yellow is produced, by mingling green and violet, a bright blue," &c.

The illusion of this doctrine is so singular, and it is so remarkable an instance of the involution of truth and error, as to merit a particular exposition.

It is true that there are but three primary colors; but green and violet may be composed, and therefore they are not primary. It is true that the green and red rays of the prism mingled may produce a yellow, but for no other reason than because yellow, which is a component of green and accompanies the red, is in excess or predominant in the mixture; for red and green, in due subordination, neutralise or extinguish each other, and produce a colorless ray. It is equally true that green and violet mingled produce a blue, because in these colors blue occurs twice and predominates over the neutrality of the mingled rays. Upon the same principle may the entire doctrine of this author be confuted, except only with respect to the number of the primary colors,

EXPERIMENTS III. & IV.

If instead of a spot, as in the foregoing experiments, an O or small circle be viewed, as before, through the convex lenticular prism, two concentric annular spectra resembling the above will appear; and if two or more concentric circles be so viewed, the number of the annular spectra appearing, will, by an effect equally beautiful and surprising, be double the number of the circles viewed, in consequence of the circles being circularly refracted.

EXPERIMENT V.

That such is the cause we may be convinced, by, viewing in like manner a narrow circle circumscribing a broad spot [Fig. 3.]; in which case the single iris resulting from the spot will appear between the double iris of the circle: there are, therefore, a double incidence and refraction produced; the one prismatical or angular, the other orbicular or circular; whence the magnitude of the spectra of this instrument in comparison with those of the common prism.

Fig. 3.

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EXPERIMENT VI.

If a circle of any diameter exceeding the field of vision of the chromascope, having a spot at the centre, be viewed in the manner of the last experiment, but with the tube of the chromascope gradually raised till the lenticular prism is about the height of the diameter of the circle above the spot at its centre, a circular spectrum will appear expanding as the instrument rises, but not two irises as in the above experiments, because the second iris, being beyond the field of vision and angle of refraction of the instrument, never enters it.'

The visible spectrum of this experiment is, however, more brilliant and beautiful than those of the smaller circles are, owing to a more perfect refraction of the object.

EXPERIMENT VII.

Upon repeating the last experiment, changing the object for one consisting of a number of concentric circles variously distinguished, a like number of spectra will be produced within that of the spot, succeeding one within another as the instrument rises, but in the inverted order of the object; the cause of which is evident from the foregoing experiments, from which it is apparent also that this experiment may be extended to any number and variety of circles.

EXPERIMENT VIII.

Let a dark circle or circles of any diameter exceeding that of the

'See Experiment XXI.

lenticular prism be formed (e. g. of 18 inches or more, and an inch in breadth) upon, or suspended against, a white wall or screen‚1 so that the light may fall upon it; and let the chromascope be adjusted concentrically to the circle in a horizontal position at right angles with the wall, and at a distance therefrom about equal to the diameter of the circle to be viewed. Then, looking through the chromascope, a beautiful colored iris will appear, which, for the reason before given, will be single."

If in this experiment the lenticular prism be removed from the chromascope and used in the above manner alone, so that the field of vision may be extended, by bringing the eye nearer to the prism, and the object be then viewed at a greater distance, an orbicular spectrum of greater magnitude and beauty will be produced.

REMARKS. In the latter mode of the present experiment the object may be placed upon the floor, or in any other convenient position to be viewed, observing only that the external ground of the object be sufficiently extended to render its spectrum distinct. It is evident also that this experiment is almost unlimited with respect to the magnitude of the object, and that Experiment VII. may be combined within it in a boundless variety of ways that fancy may dictate.

EXPERIMENT IX.

Let a spiral be formed of any number of involutions; then adjust the chromascope to its centre as in Experiment I., and looking through the instrument, gradually elevate it, when the spiral will appear refracted into two involved spiral irises; and if the figure be barbed or arrow-headed at either end, an arrow-head will be found at the central end of one of the involved irises, and at the external extreme of the other; the reason of which is apparent from Experiments V. and VII.

EXPERIMENT X.

Again, if a spiral of a diameter exceeding that of the chromascope be viewed, as in the last experiment, a single spiral iris will appear as in Experiment VI.; and if a head be formed at the outward extremity of the figure, it will appear at the centre of the spectrum. And if the figure be drawn to represent a serpent it will seem as the instrument rises to uncoil itself, and will be beautifully variegated with colors.

EXPERIMENT XI.

Upon viewing in the manner of Experiment I, a black spot å of an inch in diameter upon a white ground, at about three inches distant from the lenticular prism, a beautiful blue circle, inscribed with white and circumscribed by black, will be produced; and if a similar white spot upon a black ground be viewed in the same manner, a red and yellow circle, inscribed with black and circumscribed by white, will appear. In these experiments the external boundary, or half of the spectrum, lies out of the field of vision.

EXPERIMENT XII.

By varying the colors of the ground and spot in the last experiment, circles of any required color may be obtained; e. g. A black spot upon a yellow ground yields a circle of green, &c.

REMARKS. The preceding experiments illustrate sufficiently the powers of the chromascope with reflected light, and they have been selected as simple, general, and progressively conducive to an infinite variety of others: it is, however, upon the agency of transmitted light that the principal and most striking effects of the instrument depend, as evinced by the following experiments :

EXPERIMENT XIII.

When a beam of the sun or other strong light, admitted into a darkened chamber by an opening in a window shutter of the diameter of the tube, is passed through the convex prism in the direction of the tube, it converges to a focus, forming a cone of light, whence it diverges over a cone of shade.

If the light be received on a sheet of white paper, where it first totally emerges from the prism, the circle of light on the paper will be bordered with red. If the paper be withdrawn beyond the focus, the circle will be bordered with blue; and at the intermediate point or focus the two circles and their borders will coincide.

Beyond the focus the circle diverges into a ring or bow, which expands diametrically in proportion to its distance from the prism, the breadth of the ring also increasing with its diameter, and the colors which constitute it diverge and cross each other at a certain distance from the focus, the blue and yellow braids of light first crossing between two and three feet from the prismatic lens.