Theory of Illuminating Apparatus. By Dr. H. E. Fripp. 521 



similarly the concave mirror will produce a more intense light 

 whenever the light-source has a hmited extent (e. g. flame of a 

 lamp), provided that the source of light and the object stand to 

 each other in the relations of conjugate foci (as in WoUaston or 

 Brewster " condensers "). 



Fig. G. 



Figs. 4 -G, showing action of concave and plane mirror compared. 

 Fig. 4, Mirror inclined at 35°. 

 Fig. 5, „ „ 40°. 



Fig. 6, „ „ 45°. 



/ p, Focal plane of object. 

 a b. Aperture of diaphragm. 

 C D, Concave mirror. 

 A B, Plane mii-ror. 



s t. Converging beam on plane mirror reflected to fp = 20°. 

 s' t'. Diverging beam on concave mirror reflected to fp = 20°. 

 Supposing sufiiciLnt extent of light-source (sky, e. g.), and sufHoient min-or 

 surface, the plane mirror gives an equal illumination with concave mirror. 



N.B. The conjugate focus of concave mirror varies with inclinations of mirror 

 towards axis of instrument. 



The diaphragm aperture determines the angle of convergence within limits 

 prescribed hy extent of light-source and mirror surface. 



It must then be accepted as a general proposition that the 

 illumination of any and every single point in the field of vision is 

 of necessity always effected by converging light. The illuminating 

 rays cross each other at the plane of the object when the plane 

 mirror as well as when any concave mirror or condensing lens is 

 used. And it is inconceivable that such directions should be given 

 (and called excellent! upon Sir D. Brewster's authority) as may 

 be found in 'The Microscope,' by Jabez Hogg, 1854, page 51, as 

 follows: " Only a portion of the object should be viewed at one 

 time, and every other part excluded. The light which illuminates 

 that part should be admitted through a small diaphragm (!) — if in 

 the day-time close a portion of the \dndoiv-sliutters " (!). The 

 customary manner of speaking of parallel or diverging rays as 

 being employed to illuminate according to circumstances (!) the 



