Theory of Illuminating Aj^paratus. By Dr. II. E. Frijyp. 523 



emitted from the surface of A B in rays variously converged upon 

 the object ; and -whether the rays received by A B reach it by 

 direct incidence or indirectly (that is, after previous reflections or 

 refractions) does not signify, as there is here no question of the 

 manner in which the rays incident from the original light-source 

 are directed upon the mirror, neither any question of the extent of 

 the light source. The two sets of lenses A B C D and A' B' C D' 

 have obviously no other eflfect than to occasion various changes in 

 the course of the rays during their transference from the hght- 

 source to the object. As before explained, the whole effect consists 

 in producing an image of the flame in direct contact with the object ; 

 and in considering the question of intensity of illumination it must 

 be borne in mind that an image of the light -source brought to the 

 plane of object upon which the instrument is focusscd, is by the 

 operation of the lenses of the Microscope brought up to the eye. 

 The brightness is thus increased, because the light is made by 

 optical means to touch the retina, not because it is "condensed " on 

 the object. 



It may be laid down as a general principle that any plane 

 below the diaphragm (i. e. section across the hght-cone), or even 

 the diaphragm aperture itself, may be taken as the illuminating 

 surface, whatever changes may have occurred previously in the 

 course of the incident rays from refraction or reflection. 



Putting now the case where the magnitude of incident light- 

 cone is not regulated by a diaphragm aperture, and where the inter- 

 position of a collecting lens brings about a greater convergence of 

 rays, and under circumstances (where the aperture of light-cone is 

 not determined by the mirror) a large illuminating cone : it is 

 always possible to place the lens in such a position that its whole 

 surface should deliver hght upon the point i?, Fig. 9. The 

 effect produced in this case is the same as when hght is reflected 

 from a mirror large enough to subtend as a base the cone whose 

 outlines are formed by prolonging the outhnes of the cone dehvered 

 by the lens until they meet the mirror. The illumination given 

 by the lens would be proportioned to its proximity or distance from 

 the object ; but an equal hght would be gained by increase of 

 mirror surface. It is obvious, however, that a practical advantage 

 is obtained by the collecting lens when the light-source is of limited 

 extent, and especially when — which is the more common case — 

 both mirror and light-source are limited. Yet, as a matter of 

 theory, the maximum of light that can be obtained by any " con- 

 denser " cannot exceed that which a sufficiently large mirror and 

 expanse of light-source (sky light) would produce. 



Thus it appears that an illuminating apparatus of whatever 

 kind is efficient in two ways only. First, by equalizing the inten- 

 sity of the rays over the whole area of illumination ; secondly, by 



