INTRODUCTION TO OPTI 

 Fig. 15. 



Fig. 17. 



in this focus will take fire; for rays of light cannot be concentrated 



without accumulating a proportional quantity of heat : hence con 



cave mirrors have obtained the name of 



burning 1 mirrors. If a burning taper 



be placed in the focus (fig. 17), the 



ray which falls in the direction of the 



axis of the mirror will be reflected back 



in the same line ; but two other rays, 



drawn from the focus, and falling on the 



mirror at B and F, will be reflected to 



A and E. Therefore the rays which 



proceed from a light placed in the focus 



of a concave mirror fall divergent upon 



it, and are reflected parallel : it is 



exactly the reverse of the former figure, in which the sun's rays fell 



parallel on the mirror, and were reflected to a focus. In .other words, 



when the incident rays are parallel, the reflected rays converge to a focus ; 



when the incident rays proceed from the focus, they are reflected parallel : 



this is a very important law of optics. We have said that the image was 



formed in the focus of a concave mirror, yet glass concave mirrors 



are often seen, where the object is represented within the mirror, in the 



same manner as in those which are convex. This is the case only 



when the object is placed between the mirror and its focus ; the image 



then appears magnified behind, or within the mirror. 



SECTION II. On Refraction and Colours. 



REFRACTION is the effect which transparent mediums produce on light in 

 its passage through them. Opaque bodies reflect the rays, and transpa- 

 rent bodies transmit them ; but it is found that if a ray, in passing from 

 one medium into another of different density, fall obliquely, it is turned 

 out of its course. The power which causes the deviation of the ray is not 

 fully understood, nor completely ascertained ; but the appearances are 

 the same as if the ray (supposing it to be a succession of moving particles, 

 which is for this purpose the most convenient way of considering it) were 

 attracted by the denser medium more strongly than by the rarer. Let us 

 suppose the two mediums to be air and water: when a ray of light passes 

 from air into water, it appears to be more strongly attracted by the 

 latter. If then a ray, A B (Jig. IS), fall perpendicularly on water, the 



