are as nearly proportioned in the figure as the PLATE small space pq would admit of.*

XIX.

If all these colours be blended together again, they will make a pure white; as is proved thus. Take away the paper on which the colours pq fell, and place a large convex glass D in the · rays f, g, h, &c. which will refract them so as to make them unite and cross each other at W; where, if a white paper be placed to receive them, they will excite the idea of a strong lively white. But if the paper be placed farther from the glass, as at rs, the different colours will appear again upon it, in an inverted order, occasioned by the rays crossing at W.

As white is a composition of all colours, so black is a privation of them all, and, therefore, properly, no colour.

B and

D 60;

D and

Let two concentric circles be drawn on a Fig. 6. smooth round board ABCDEFG, and the outermost of them divided into 360 equal parts, or degrees: then, draw seven right lines, as OA, B, &c. from the centre to the outermost circle; making the lines A and B include 80 degrees of that circle; the lines OC 40 degrees; O C and OE 60; E and OF 48; OG and OA 45. Then, between these two circles, paint the space AG red, inclining to orange near G; GF orange, inclining to yellow near F; FE yellow, inelining to green near E; ED green, inclining to blue near D; DC blue, inclining to indigo near C; CB in

OF and OG 27;

* The above measures of the coloured spaces in the prismatic spectrum do not always hold; nor are these spaces constantly proportional. There is an evident fallacy in Newton's experiment upon this subject, for when the mean refractive power of glass is given, we cannot thence infer its power of dispersion. See APPENDIX on Achromatic telescopes.-E. E‹Ð.

make a

All the digo, inclining to violet near B; and BA prismatic colours violet, inclining to a soft red near A. This blended done, paint all that part of the board black together, which lies within the inner circle: and putwhite. ting an axis through the centre of the board, let it be turned very swiftly round that axis, so as the rays proceeding from the above colours may be all blended and mixed together in coming to the eye; and then the whole coloured part will appear like a white ring, a little greyish; not perfectly white, because no colours prepared by art are perfect.

Transpa

come

opaque, if

Any of these colours, except red and violet, may be made by mixing together the two contiguous prismatic colours. Thus, yellow is made by mixing together a due proportion of orange and green; and green may be made by a mixture of yellow and blue.*

All bodies appear of that colour whose rays they reflect most; as a body appears red when it reflects most of the red-making rays, and absorbs the rest.

Any two or more colours that are quite transparent coPoveste parent by themselves, become opaque when put lours betogether. Thus, if water, or spirits of wine, be put toge tinged red, and put in a phial, every object seen through it will appear red; because it lets only the red rays pass through it, and stops all the rest. If water or spirits be tinged blue, and put in a phial, all objects seen through it will ap

ther.

* Our author is here mistaken, as pure yellow cannot be composed by mixing together any proportion of green and orange. It is on this account that the celebrated Tobias Mayer, maintains, in opposition to Newton, De affinitate Colorum, in his Opera Ine. dita, published by Litchenberg), that there are only three prima. ry colours, viz. red, yellow, and violet, because every other colour may be composed of a proper proportion of any two of these, while they themselves are simple, and incapable of being com. pounded.-E. ED.

pear blue, because it transmits only the blue rays, and stops all the rest. But if these two phials be held close together, so as both of them may be between the eye and object, the object will no more be seen through them than through a plate of metal; for, whatever rays are transmitted through the fluid in the phial next the object, are stopped by that in the phial next the eye. In this experiment, the phials ought not to be round, but square; because nothing but the light itself can be seen through a round transparent body, at any distance.

As the rays of light suffer different degrees of refraction by passing obliquely through a prism, or through a convex glass, and are thereby separated into all the seven original or primary colours; so they also suffer different degrees of refraction by passing through drops of falling rain; and then, being reflected toward the eye, from the sides of these drops which are farthest from the eye, and again refracted by passing out of these drops into the air, in which refracted directions they come to the eye; they make all the colours to appear in the form of a fine arch in the heavens, which is called the rain-bow.

There are always two rain-bows seen toge- PLATE ther, the interior of which is formed by the rays XIX. ab, which falling upon the upper part b, of the Fig. 7. drop bed, are refracted into the line be as they enter the drop, and are reflected from the back of it at c, in the line cd, and then, by passing out of the drop into air, they are again refracted at d; and from thence they pass on to the eye at e; so that to form the interior bow, the ray suffers two refractions, as at b and d; and one reflection, as at c.

PLATE
XIX.

Fig. 8.

The exterior bow is formed by rays which, suffer two reflections, and two refractions; which is the occasion of its being less vivid than the interior, and also of its colours being inverted with respect to those of the interior. For, when a ray ab falls upon the lower part of the drop bede, it is refracted into the direction be by entering the drop; and passing on to the back of the drop at c, it is thence reflected in the line cd, in which direction it is impossible for it to enter the eye at f: but, by being again reflected from the point d of the drop, it goes on in the drop to e, where it passes out of the drop into the air, and is there refracted downward to the eye, in the direction ef.*

* A rain-bow is sometimes seen in the evening, when the moon is near her full, formed by the refraction of her light through the drops of rain, and is called the Lunar Iris.-E. ED,

283

LECTURES VIII AND IX.

The description and use of the Globes, and
Armillary Sphere.

restrial

IF a map of the world be accurately de- The terlineated on a spherical ball, the surface there- globes. of will represent the surface of the earth; for the highest hills are so inconsiderable with respect to the bulk of the earth, that they take off no more from its roundness, than grains of sand do from the roundness of a common globe; for the diameter of the earth is 8000 miles in round numbers, and no known hill upon it is 3 miles in perpendicular height.

the earth's

That the earth is spherical, or round like a Proof of globe, appears, 1. From its casting a round being gloshadow upon the moon, whatever side be turned bular. toward her when she is eclipsed; 2. From its having been sailed round by several persons; 3. From our seeing the farther, the higher we stand; 4. From our seeing the mast of a ship, while the hull is hid by the convexity of the

water.

without

any one's

The attractive power of the earth draws all And that terrestrial bodies toward its centre; as is evi- it may be peopled dent from the descent of bodies in lines per- on all sides pendicular to the earth's surface, at the places whereon they fall; even when they are thrown being in off from the earth on opposite sides, and con- danger of falling asequently in opposite directions. So that the way from earth may be compared to a great magnet roll- it.