xeii INTRODUCTION TO OPTICS. 



shower of rain, every drop of which acts as a prism, in separating the 

 coloured rays as they pass through it. 



1 The sun's rays may be collected to a focus by a lens in the same 

 manner as they are by a concave mirror: in the first, the rays pass 

 through the glass, and converge to a focus behind it; in the latter they 

 are reflected from the mirror, and brought to a focus before it. A lens, 

 when used for this purpose, is called a burning glass ; and if, when 

 the sun shines bright, a piece of paper be held in the focus of the rays, 

 it will take fire. This experiment succeeds best with brown or any 

 dark-coloured paper ; for though it is true that the lens collects an equal 

 number of rays to a focus, whether the paper held there be white or 

 coloured, the white paper appears more luminous in the focus, because 

 most of the rays, instead of entering into the paper, are reflected 

 by it ; and this is the reason that the paper is not burnt ; whilst, on the 

 contrary, the coloured paper, which absorbs more light than it reflects, 

 soon becomes heated and takes fire. 



It is supposed that the tendency to absorb or reflect rays depends on 

 the arrangement of the minute particles of the body, and that the diversity 

 of arrangement renders some bodies susceptible of reflecting one coloured 

 ray, and absorbing the others ; whilst other bodies have a tendency to 

 reflect all the colours, and others again to absorb them all. A body 

 appears to be of the colour which it reflects ; as we see it only by reflected 

 rays, it can appear but of the colour of those rays. Thus grass is green, 

 because it absorbs all except the green rays : it is, therefore, these only 

 which the grass and trees reflect to our eyes, and which make them 

 appear green. The sky and flowers, in the same manner, reflect the 

 various colours of which they appear to us : the rose, the red rays ; the 

 violet, the blue ; the jonquil, the yellow, &c. If you imagine that these 

 are the permanent colours of the grass and flowers, you are mistaken. 

 Whenever you see those colours, the objects must be illuminated; and 

 light, from whatever source it proceeds, is of the same nature, composed 

 of the various coloured rays, which paint the grass, the flowers, and every 

 coloured object in nature. Objects in the dark have no colour, or are 

 black, which is the same thing. You can never see objects without light. 

 Light is composed of colours, therefore there can be no light without 

 colours ; and though every object is black, or without colour in the dark, 

 it becomes coloured as soon as it becomes visible. 



An object placed in a coloured ray of light which has been refracted 

 by a prism, will appear of the colour of the ray in which it is placed. A 

 sheet of white paper will take all the colours indifferently, but a coloured 

 body will appear most brilliant when placed in the ray which it naturally 

 reflects. But though bodies, from the arrangement of their particles, 

 have a tendency to absorb some rays and reflect others, yet they are not 

 so perfectly uniform in their arrangement as to reflect only pure rays of 

 one colour, and perfectly absorb the others. A body reflects, in great 

 abundance, the rays which determine its colour, and the others in a 

 greater or less degree, in proportion as they are nearer or farther from 

 its own colour, in the order of refrangibility. 



Bodies which reflect all the rays are white ; those which absorb them 

 all are black. Between these extremes they appear lighter or darker, in 

 proportion to the quantity of rays they reflect or absorb. A rose is of a 

 pale red : it approaches nearer to white than black, it therefore reflects 

 rays more abundantly than it absorbs them. Pale-coloured bodies reflect 

 all the coloured rays to a certain degree, which produces their paleness, 



