HARDWICK&S SCIENCE-GOSSIP. 



249 



If two or more parallel rays fall on such a mirror, 

 it follows that they will continue parallel after reflec- 

 tion. 



If tvvo or more convergent or divergent rays fall 

 upon such a mirror, they will continue to converge, or 

 to diverge, after reflection, just as if their lines of 

 direction had not been altered. Thus, in Fig. 156 the 

 converging rays L L continue to converge after reflec- 

 tion precisely as they did before it, and come to a 

 focus at the point E. Had their direction not been 

 changed, they would have come to a focus at e', as 

 shown by the dotted lines ; E and e' being equidistant 

 from D d, but on opposite sides of A B. The reflected 



line passing through both c and D is the " principal 

 axis." Any line which passes through c, and through 

 any other point of the mirror than its central point, D, 

 is a "secondary axis." There can, therefore, be only 

 one principal axis, while the number of secondary 

 axes is unlimited. These expressions must be care- 

 fully distinguished from another, which I shall often 

 have to make use of, viz. "axis of the microscope." 

 The latter expression denotes an imaginary straight 

 line, passing through the centre of the eye-piece, 

 along the centre of the tube, through the centre of 

 the object-glass, and indefinitely onwards. When 

 the mirror is so placed that this line passes through 



- — "^ 



Fij- 157- 



ray is not quite equal in brilliance to the incident ray, 

 a portion of the light being absorbed by the reflecting 

 surface, but the amount of loss diminishes as the 

 obliquity of the incident ray is increased. 



We now turn to the Concave Mirror. In Fig. 157, 

 A B represents the section of a concave mirror, made 

 through its central point, D. This mirror is a portion 

 of the inner surface of a hollow sphere having the 

 centre c. (The reader must not suppose that the 

 mirror of his microscope has so pronounced a curva- 

 ture as that shown in the figure. The real thing is 

 a very small portion of a very much larger sphere.) 

 C is the mirror's " centre of curvature." The straight 



its central point, D, the mirror is said to be in the 

 axis of the microscope ; and if the surface of the 

 mirror be turned upwards towards the stage, so that 

 its centre of curvature, c, be also in the same line 

 the principal axis of the mirror, and the axis of the 

 instrument will coincide, and the line c D, if produced 

 upwards from c, will pass direct to the eye-piece of 

 the microscope. In practice this latter position of 

 the mirror cannot be employed. It is, however, 

 desirable always to have the point D accurately in the 

 axis of the microscope, unless a pencil of oblique rays 

 is intentionally to be employed. In passing, I may 

 remark that an object on the stage, when under 



