Illumination icith Plane Mirror. By Dr. 11. E. Fripp. 743 



time render tliein virtnally self-luminous, strengthens the propo- 

 sition that the illuminating power of any light-reJSccting apparatus 

 is measured hy the nunibei- and direction of rays which fall upon 

 the object (placed on the Microscope stage) from the mirror or lens 

 surface next to it : and that the specific intensity of the light can 

 never he greater than that of the primary light source. This 

 problem is reduced to its simplest possible form in the case of the 

 plane mirror. We have but to remember that each constituent 

 point of its surface is independent of every adjacent point, so far as 

 incidence and reflection of the light rays are concerned. And it 

 remains only to discuss the right application of the law of reflection 

 under given circumstances, and to note that the physical constitu- 

 tion of ordinary daylight enables it to fuUil the conditions required 

 for illumination with the plane mirror. 



It is not denied that out of a countless number of rays im- 

 pinging from all sides on the surface of a mirror freely exposed to 

 daylight, and consequently reflecting them on all sides according to 

 their several lines of incidence, the larger proportion falls with 

 parallel incidence. But it is contended that ]iarallel beams are 

 reflected as such, and — for that very reason — their component rays 

 do not touch the object, but pass by and outside of it. It is further 

 contended that every ray which docs reach the object must approach 

 from without in a direction determined by the position of the point 

 or surface-element of the mirror where the reflection takes place ; 

 that is to say, on the relation of this position to the object on one 

 side and the light source on the other. 



The demonstration of these points is offered in the accompanying 

 diagrams. 



It will be granted that obliquity of incidence must always give 

 rise to obliquity of rejlection. 



Further, it will appear from consideration of the relative posi- 

 tion of object, mirror, and light source (due to mechanical arrange- 

 ment of the Microscope), that light must always fall obliquely on 

 the mirror in order to strike the object, whatever be the size or 

 inclination of the mirror. 



The problem of converging illumination is therefore demon- 

 strated when it is shown that rays incident upon constituent 

 surface elements of the mirror outside of its centre do incline, after 

 reflection, towards an axial line above that centre. This axial lino 

 coincides with the axis of the TMicroscope when the mirror is set for 

 "central illumination," but when the mirror is moved into position 

 for oblique ilhimination, the axial Hnt^ is also obli(iuo and coincident 

 with an imagiufiry lino drawn Irom ecntro of the mirror to the 

 several points of the ol)jecton which illuminating })encils fall. But 

 those rays alone will reach the object which full with tho necessary 

 incidence and reflection. 



