When the object is more remote from the mirror than its centre of concavity, C, the image will be less than the object, and between the object and the mirror ; when the object is nearer than the centre of concavity, the image will be more re mote, and bigger than the object : thus, if D E be the object, e d will be its image ; for as the object recedes from the mirror, the image approaches nearer to it ; and as the object approaches nearer to the mirror, the image recedes further from it, on account of the lesser or greater divergency of the pencils of rays which proceed from the object ; for the less they diverge, the sooner they are con verged to points by reflection ; and the more they diverge, the further they must be reflected before they meet. If the ra dius of the mirror's concavity and the distance of the object of it be known, the distance of the image from the mirror is found by this rule : Divide the product of the distance and radius by double the dis tance made less by the radius, and the quotient is the distance required. If the object be in the centre of the mirror's concavity, the image and object will be coincident, and equal in bulk.
If a man place himself directly before a large concave mirror, but further from it than its centre of concavity, he Will see an inverted image of himself in the air, between him and the mirror, of a less size than himself. And if he hold out his hand towards the mirror, the band of the image will come out towards his hand, and coincide with it of an equal btilk, when his hand is in the centre of concavity ; and he kill imagine he may shake bands with his image. If he reach his hand fur ther, the hand of the image will pass by his hand, and come between it and his body ; and if he move his hand towards either side, the hand of the image will move towards the other ; so that what ever way the object moves, the image ‘vill move the contrary way. A by-stand er will see nothing of the image, because none of the reflected rays that form it enter his eyes.
The images formed by convex specula are in positions similar to those of their objects ; and those also formed by con cave specula, when the object is between the surface and the principal focus : in these cases the image is only imaginary, as the reflected rays never come to the foci from whence they seem to diverge. In all other cases of reflection from con cave specula, the images are in positions contrary to those of their objects, and these images are real, for the rays after re flection do come to their respective foci. These things are evident from what has gone before. See Mallon.
" Of colours and the different refrangi bility of light." The origin of colours is owing to the composition which takes place in the rays of light, each hetero geneous ray consisting of innumerable rays of different colours ; this is evident from the separation that ensues in the well-known experiment of the prism. A
ray being let into a darkened room (fig. 11) through a small round aperture, z, and falling on a triangular glass prism, x, is by the refraction of the prism considerably dilated, and will exhibit on the opposite wall an oblong image, a b, called a spec trum, variously coloured, the extremities of which are bounded by semicircles, and the sides are rectilinear. The colours are commonly divided into seven, which, however, have various shades, gradually intermixing at their juncture. Their or der, beginning from the side of the re fracting angle of the prism, is red, orange, yellow, green, blue, purple, violet. The obvious conclusion from this experiment is, that the several component parts of solar light have different degrees of re frangibility, and that each subsequent ray in the order above mentioned is more re frangible than the preceding.
Asa circular image would be depicted by the solar ray unrefracted by the prism, so each ray that suffers no dilatation by the prism would mark out a circular im age, y. Hence it appears, that the spec-' trum is composed of innumerable cirClea of different colours. The mixture, there fore, is proportionable to the number of circles mixed together (fig. 12) ; but all such circles are mixed together, whose centres lie between those of two contin gent circles, consequently the mixture is proportionable to the interval of those centres, i. e. to the breadth of the spec trum. If therefore the breadth can be diminished, retaining the length of the rectilinear sides, the mixture will be lessened proportionably, and this is done by the following process.
At a considerable distance from the hole, 2, place a double convex lens, A B (fig 13), whose focal length is equal to half that distance, and place the prism x, behind the lens ; at a distance behind the Lens, equal to the distance of the lens from the hole, will be formed a spectrum, the length of whose rectilinear sides is the same as before, but its breadth much less ; for the undiminished breadth was equal to a line subtending, at the distance of the spectrum from the hole, an angle equal to the apparent diamater of the sun, together with a line equal to the diameter of the hole ; but the reduced breadth is equal to the diameter of the hole only ; the image of the hole formed by the lens at the distance of double its focal length, is equal to the hole ; there fore, its several images in the different kinds of rays are equal to the same, i. e. the breadth of the reduced spectrum is equal to the diameter of the hole.