742 Transactions of the Society. 



XXI. — On Daylight Illumination with the Plane Mirror. 



An Appendix to Part I. of the " Theory of Illuminating 

 Apparatus." * 



By the late Dr. H. E. Fripp, Ex.-off. F.R.M.S. 



(Read l-ith January, 1880.) 



In my paper on the theory of illuminating apparatus, published in 

 the last volume of the Journal, I referred at some length to a 

 doctrine, which, being well accredited by scientific men abroad, did 

 not occur to me as a possible stumbling-block to the acceptance of 

 arguments based thereon, until I learned how much it was at 

 variance with the teaching and belief of microscopists in this 

 country. It was, namely, contended that an object placed on the 

 stage of the Microscope is always and necessarily illumined by a 

 converging pencil when daylight is reflected upon it from the plane 

 mirror. In English handbooks of the Microscope it is, on the 

 contrary, assumed without question that the illuminating pencil is 

 derived from a parallel beam of rays incident on the mirror whether 

 plane or concave. Now since the conclusions, theoretical and prac- 

 tical, deduced from such widely opposed premises cannot but be as 

 contradictory as the premises themselves, while, moreover, they 

 cannot both be true, it is desirable as well as important, in a 

 scientific point of view, that these antagonistic beliefs should be 

 brought to the final arbitrement of fact. The question is one which 

 optical science is perfectly competent to determine, proof or dis- 

 proof of either proposition being readily drawn from consideration 

 of first principles or from experimental tests. In the hope that a 

 more explicit statement of the rationale of " converging light " may 

 bring it more fairly under the notice of those who may be disposed 

 to give this doctrine due attention, I now present a short summary 

 of the grounds upon which it rests. 



The transference of light, in an optical sense, from one point or 

 surface to another is efiected either by reflection or refraction. And 

 in discussing the function of the mirror our sole appeal is to the 

 law of reflection, just as the law of refraction would be appealed to 

 if the action of the various lenses of an illuminator were in question. 

 But in connection with this transference of light by means of reflec- 

 tor or refractor an interesting problem occurs in estimating the 

 illuminating eff'ect of the difierent surfaces which consecutively take 

 the place of the primary light source, ending with the last reflecting 

 or refracting surface brought into action (as in compound illumina- 

 tors). All that we know of the property of light in rendering 

 visible to the eye material particles upon which rays impinge in 

 such direction as to enter the pupil when reflected, and so for the 



* See this Journal, ii. (1879) p. 503. 



