250 



HARD WICKE'S SCIENCE- G OS SIP. 



observation, must, of course, be in the axis of the 

 instrument, otherwise the eye could not see it. 

 When, therefore, we bring rays to a focus on the 

 object, that focus will be in the said axis also. 



In Fig. 158 rays of light, L L L, which are parallel 

 both to c D, the principal axis, and to each other, fall 

 upon the mirror, and are reflected to a point F, in the 



If the microscopist could make use of the " principal 

 focus," it would often be advantageous to do so, 

 because the rays are so symmetrically reflected, and 

 so nearly brought to an exact focus ; but he cannot. 

 The object on the stage would need to coincide in 

 position with the point F, or nearly so. The surface 

 of the mirror would therefore be upwards towards the 



M 





.-'.<'■■:-'' 



Fig. 159. 





said 

 rays, 

 apex 

 focus 



Fig. 160. 



axis, mid-way between c and d. Such parallel 

 after reflection, form nearly a true cone, whose 

 is at F. This point F is called the "principal 

 " of the mirror. 



Fig. 161. 



stage, the principal axis being coincident with the 

 axis of the microscope. Further, the incident rays 

 would need to be parallel to the principal axis, CD; 

 the lamp and bull's-eye would have, therefore, to 

 be placed in line with the eye-piece, and parallel 

 rays to be directed through, and around, the tube, 

 on to the mirror. Of course, this is impracticable, 

 and so the " principal focus " cannot be employed. 



In Fig. 159 the rays lll are parallel to each other, 

 as before ; but instead of being parallel to the prin- 

 cipal axis, c D, they form an angle of 30 with it ; 

 that is to say, the angle of incidence of the central 

 ray at the point D is 30 . But it may be asked, 

 How are angles of incidence and reflection to be 

 estimated in the case of a curved surface ? Well, a 

 curved surface may be thought of, as made up of an 

 infinite number of minute plane mirrors placed side 

 by side, so that if a perpendicular be drawn to any 

 point, reflection from that point will follow the 

 same law as in the case of the plane mirror (Fig. 155). 

 It conveniently happens that the required perpen- 

 dicular to any point in our concave mirror is the 

 line drawn from the "centre of curvature " to that 

 point. The dotted lines in Figs. 158 to 161 represent 

 these radial lines or perpendiculars. In Fig. 159 we 

 note (1) that the parallel rays L after reflection, are 



