in its original direction to F ; the consequence will be, that it will pursue an 

 intermediate course to E. 



If a coin be placed in a basin, so that on standing at a certain distance it be 

 just hid from the eye of an observer by the edge of the basin, and then water be 

 poured in by a second person, the first keeping his position, as the water rises 

 the coin will become visible, and will appear to have moved from the side to the 

 middle of the vessel (fig. 15). 



These facts lead to an important axiom in optics ; namely, that we see every- 

 thing in the direction of that Line in which the rays approach us last. Hence, 

 the sun is seen several minutes before it comes to the horizon, and as long after 

 it has sunk beneath it, because its rays strike first upon the atmosphere, and are 

 by it refracted, or bent towards the earth (fig. 16). 



In passing through a pane of glass the rays suffer two refractions, but which, 

 being in contrary directions, produce nearly the same effect as if no refraction 

 kad taken place. A A, fig. 17, represents a thick pane of glass seen edgeways. 

 When the ray B approaches the glass at c, it is refracted to D ; at that point, 

 returning into the air, it is again refracted, but in a contrary direction, and in 

 consequence proceeds to E. But this is the case only when the two surfaces 

 are parallel to each other j if they are not, the two refractions may be made ( in 

 the same direction. Thus, when the parallel rays (fig. 18) fall on a piece ' of 

 glass having a double convex surface, that ray only which falls in the direction 

 of the axis of the lens is perpendicular to the surface : thp- nt.hr.r rays falling 

 obliquely are refracted towards the axis, and meet at a point beyond the ic, nn 

 called its focus. 



Figs. 19 to 23 represent sections of lenses of various forms, having different 

 refractive properties. The property of those which have a convex surface is to 

 collect the rays of light to a focus ; and of those having a concave surface to 

 disperse them. The rays falling on the concave lens, fig. 25, will each be at- 

 tracted towards its thicker extremities, both on entering and quitting it ; and 

 will, therefore, by the first refraction be made to diverge to A c, and by the 

 second to d e The ray B, falling perpendicularly on the axis of the lens, suffers 

 no refraction. 



Lenses which have one side flat, and the other convex or concave, as figs. 19 

 and 20, are called plano-convex and plano-concave. The focus of the former is 

 at the distance of the diameter of the sphere, of which the convex surface of the 

 lens forms a portion, as shown at fig. 26. 



Fig. 24 represents a section of a prism, the principal use of which is to enable 

 us to decompose the rays of light. 



Decomposition Of Light- White light, as emitted from the sun, or other 

 luminous body, is found to be composed of seven different kinds of light ; namely, 

 red, orange, yellow, green, blue, indigo, and 'violet ; and this compound substance 

 may be decomposed or separated into its elementary parts. 



Fig. 27, represents a prism, so placed, that a beam of light, admitted by a 

 small aperture, A, falls upon it, and being refracted is thrown upon the screen 

 B c, forming an oblong image called the prismatic spectrum, containing the seven 

 colours already named. 



The Raint)OW is formed by the sur^s rays falling upon the upper parts 

 of the drops of rain, and being then, by refraction, thrown on another part of the 

 same drop, where they are again refracted and reflected to the eye, so as to pro- 

 duce the successive colours from the upper part of red, orange, yellow, green, 

 blue, indigo, and violet (fig. 28). 



The Eye Vision- Figs. 29 and 30 represent a front view and a section 

 of the eye. The eye is composed of three coats or skins, one covering the other. 

 Within the coverings of the eye-ball are contained three transparent sub- 

 stances, called humours. These different humours form a compound lens, which 

 refracts the rays of light rebounding from objects, forming an image of them 

 upon the retina, the sensation being transmitted by the optic nerve to the brain. 



Telescopes- Fig. 31 illustrates the construction and action of a refracting 

 telescope, and fig. 32, that of a reflecting telescope. The lines show the direc- 

 tion in which the rays are transmitted through the various lenses. 



Fig. 33 represents the camera olscura. This interesting optical instrument 

 consists of a convex lens A, through which the rays of any objects are admitted 

 into a darkened chamber, where they fall on a plane mirror B, placed at an angle 

 of forty-five degrees ; by this they are reflected upwards against a plate of ground 

 glass c, upon the upper surface of which the objects appear in their natural colours 



Fig. 34 represents that well known instrument, the magic lantern. 



