TilEORY OF LIGHT. 267 



medium, as from air into water, it suffers no change in its direction; but 

 when it passes obliquely, it takes a new direction towards the perpendi- 

 cular, making a sudden angle, as if broken, and this is refraction. Two 

 circumstances, therefore, influence the ray of light ; the angle at which 

 it falls, and the density of the body into which it passes. When the ray 

 passes from the denser medium into the rarer, it is again refracted, but 

 away from the perpendicular, and takes its original course, provided the 

 surface at which it goes out is parallel to the surface at which it entered. 

 When a ray strikes upon a body that is not transparent, or only 

 imperfectly so, it is in part reflected, that is, struck off again, bent back, 

 or reflected, and enters the eye, conveying to us the impression of the 

 form and colour of that object. But the expression which we have used 

 requires explanation ; for how is it that the reflected rays should convey 

 the idea of colour ? The prism is a piece of glass so formed that the rays 

 must fall obliquely on one or both of the surfaces, and suffer refraction. 

 Thus a ray striking into the prism is refracted; but all its parts are not 

 equally refracted, and as the light consists of parts differing in colours, 

 and which are differently refracted, it is divided or dissected into several 

 colours, called the prismatic colours. The spectrum, as it is termed, 

 thus formed, consists of seven colours ; that which is least refracted 

 being red, and in succession orange, yellow, green, blue, indigo, and 

 violet. If these rays be re-compounded by passing through a convex 

 lens, which, owing to the obliquity with which they fall, draws them to 

 a point, the focus of the light will be again colourless. Some modern 

 philosophers have reduced these prismatic colours of Newton to three 

 primary colours, red, yellow and blue ; contriving, by the super-position 

 of these, to produce the seven tints; while others have, on considerations 

 not easy to be disproved, held that there is not any definite number of 

 colours, but a gradation of tints from the extreme red to the extreme 

 violet. We may now understand the reason of the colour of objects. 

 When light strikes upon a body, even upon the most transparent, part 

 penetrates, part is reflected, and some part is lost. A dye is a disposition 

 given to the surface of cloth to repel some of the rays of light more than 

 the others; and the colour will be according to the ray, or the combina- 

 tion of rays, thus cast back and sent into the eye. 



And here it is natural to reflect on the variety and beauty everywhere 

 bestowed through this property of the beam of light. What a dulness 

 would have pervaded the surface of the earth if there had been only a 

 white light! The beauties of the garden and of the landscape would 

 have been lost to us. How is the beauty of the latter enhanced by the 



