518 



Transactions of the Society. 



feeble. But if the mirror be inclined forwards, backwards, or 



sidewards, at a greater angle, the outside rays of the beam fall 



Avith a certain obliquity, depending upon the size of the reflected 



Fig. 3.* 



Fig. 3, to sliow that plane and concave mirrors have au exactly similar con- 

 verging effect from the mirror upon the object. 



The effect of the curve of the concave mirror is, here, to cause parallel rays 

 incident upon it to be converged with an inclination to F P equal to that which is 

 produced by means of the plane mirror when a larger converging cone of rays 

 falls upon it ; the area of mirror surface being practically equal in both. 

 Angle of aperture, 20°. 

 Distance of mirror from F P, 4 inches. 

 Radius of curve of concave mirror, 12 inches. 

 a a, Kadii for constructing angles of incidence and reflection on concave 



miiror. 

 h b. Verticals to plane mirror. 

 Dotted lines = direction of incident light upon plane mirror. 

 Continuous lines = direction of rays incident upon concave mirror. 



beam and its freedom from obstruction by the projection of stage 

 or other impediment to free play of the light. In the small model 

 instruments in which a relatively large mirror is centrally fixed at 

 a short distance beneath the stage, the obhque illumination thus 

 obtained has a marked influence upon the delineation of many 

 objects. 



When, again, the surface of light-source is reduced less than that 

 of the mirror, the illuminating pencil falls with diverging rays upon 

 the mirror and with corresponding diminution of light intensity, 

 the same divergence and diminution of light effect continuing after 

 reflection. 



What now is the function of the diaphragm as ordinarily used 

 with the plane mirror ? It is evident from inspection of Figs. 4, 5, 6,t 



* The dotted lines, by error of the engraver, are drawn parallel with each other : 

 each line traced backwards from the object has its special angle of divergence 

 from, or convergence towards, the mirror. 



t Taken from the Handbook of N'agcli and Schwcndener. 



