Theory of Illuminating Ajjpamfus. By Dr. H. E. Fripp. 513 



appear to have agreed with the theory propounded by the autho- 

 rities of the day, as will be seen from the following extract : — 

 " AVe may mention it as a fact for which we cannot very well 

 account, that we have been able to obtain a very beautiful and dis- 

 tinct illumination by the use of an aplanatic doublet condenser, 

 receiving its rays from the ground-glass globe of the common table- 

 lamp, which would not seem to furnish any of the conditions that 

 we have dwelt on as of theoretical importance." 



In respect to the opinions expressed in the above quotations, 

 I need at present only remark that although a theory of illumina- 

 tion is partially indicated in the acceptance of Brewster's " achro- 

 matic condenser" and the principle of its mode of action, as ex- 

 plained by its inventor (which has already been criticized), the 

 suggestion of optical relation between the size of the illuminating 

 cone and the aperture of the objective, more distinctly asserted by 

 Eoss (see ante), is based upon the assumption that every objective 

 works best at its fullest aperture, which is rarely the case, and that 

 " definition " and " penetration " are always improved by intense 

 light, which is the case only when the focussing function of the 

 objective is absolutely perfect ; and even then definition becomes 

 more critical as the ivhole aperture of the objective is occupied at 

 the same moment. Sir D. Brewster's insistance on the propriety 

 of using a narrow point of light, amounts in practice, when his 

 illuminator is used, to this — that its specific intensity is first 

 equalized by being distributed over a large radiant surface, viz, of 

 the lens next to the object, and then directed from a number of 

 radiant points in lines converging on the object. The true rationale 

 of " definition," so far as brightness of illumination is concerned, is 

 indicated by Helmholtz in his essay " On the Limits of Optical 

 Capacity of the Microscope," written at a much later date. SuflS- 

 cient delineating shadow, he says, can only be got by narrowing 

 the size of the illuminating pencil, and a comparatively large illu- 

 minating cone can be applied beneath the object only when tho 

 objective admits a large incident cone. But it must be borne in 

 mind that large angular aperture is associated with high magni- 

 fying powers, and as the optical aperture of such objectives dimi- 

 nishes with each increase of magnifying power, difiraction effects 

 begin to affect the delineation. The theory of these diffraction 

 effects was not however studied or followed out to its proper con- 

 clusion by any writer of that date. Again, it has been fully 

 demonstrated that the function of large aperture is summed up in 

 the admission and transmission of diffraction pencils, caused by the 

 splitting of such pencils of light as fall upon minute details of the 

 object into difiraction pencils which do not follow the original 

 direction of the pencils thrown by the illuminator. And although 

 fresh detail is thereby introduced into the image by an objective of 



