Theory of Illuminating Aiyparaius. By Dr. H. E. Fripp. 525 



the aperture of the ilhiminating cone, be supplied with the means 

 of excluding light from any part of it. Many objects are best 

 seen when central rays are excluded, others when oblique illumi- 

 nation is employed. To regulate the illumination according to 

 requirement a second diaphragm is needed : a revolving disk or 

 sector fitted over the first with a set of diflTerently shaped stops to 

 be rotated over the openings. Any arrangement may be intro- 

 duced which suits the mechanical requirements of the instrument ; 

 but the diaphragms should be placed above the lens, and not, as is 

 generally the case, beneath it. 



In the foregoing observations the optical principles upon which 

 the various apparatus for illumination of microscopic objects by 

 transmitted light depend, have been indicated in general terms, 

 which may be thus summed up : — 



Whether concave or plane mirror, prism, collecting lens, or 

 achromatic " condenser " be employed, the cross section of the 

 illuminating beam shows a plane which is bounded either by dia- 

 phragm aperture, or by limits of reflecting or refracting apparatus, 

 and which may be considered as self-luminous in relation to the 

 object, inasmuch as each point of its surface conducts light from the 

 primary source with the same illuminating power as that source, 

 excepting the insignificant loss by reflection from the lens surfaces 

 through which the light is made to pass. The size and position of 

 the effective light-giving surface define the extent of a converging 

 cone falling upon each point of the object, which, traced towards the 

 Microscope, offers a diverging cone from each transparent point of 

 the object to the objective. The aperture of the objective defines 

 in its turn the basis of the diverging beam admitted into the objec- 

 tive, and therefore the brightness of the image formed. When 

 diffraction pencils are occasioned by the action of the object, their 

 course differs fi-om that of the ordinary pencils, and the admis- 

 sion of such diverted pencils depends on increased aperture. The 

 track of the ordinary and diftractive pencils can be observed as an 

 image of the light-giving surface at the upper focal plane of the 

 objective. And this method of examining the performance of the 

 objective shows, amongst other things, the illusory character of the 

 general belief that illumination is effected by parallel or diverging 

 as well as by converging rays. This subject and its illustration by 

 experiment and diagram is reserved for another opportunity. 



The intensity of light transmitted by any kind of " condenser " 

 cannot be greater than that which the original source yields. But 

 by the production of an intermediate illuminating surface the light 

 from the original and more distant source is brought nearer, and 

 the convergence upon the object greater (under circumstances deter- 

 minable at will) while the intensity is equalized over the whole 

 illuminating surface. 



VOL. II. 2 N 



