ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 355 



the object are admitted. The consideration of the diffraction pencils, 

 their admission or non-admission into the field, bears therefore, not 

 merely on the resolving power but on the delineating power of the 

 Microscope, i. e. on the fundamental aim of microscopical observation 

 to see things as they are. 



Those who have recognized these facts of microscopical vision, 

 know how difficult it has been to induce those who have always looked 

 upon the Microscope as essentially belonging to geometrical optics 

 not merely to recognize that it has largely passed into the domain of 

 physical optics, but even to listen to the suggestion. The difficulty 

 is not in their grasping the fact when it is once explained to them, as 

 the experiments which illustrate it are very simple ; but is of the same 

 kind as that which prevented our forefathers even investigating the 

 (to them) absurd allegation that the earth revolved on its axis.* 



(b) Coarser Objects. 



It will be useful to note for those who have not had the oppor- 

 tunity of following the continuation of the researches of Professor 

 Abbe, that they have enabled him to complete the diffraction theory. 



In his original j)aper Professor Abbe established, as we have 

 shown above, the af)plication of the theory to the delineation of 

 minute structures which by their diffractive effect produce a per- 

 ceptible breaking up of the incident (transmitted or reflected) rays, 

 the images of the coarse parts of microscopical objects (measured by 

 considerable multiples of the wave-length) being treated as depicted 

 on the ordinary dioptric method, giving what was called the " dioptric 

 image," the minute structural parts giving the " interference image." 



The further researches showed that strict dioptrical delineation, 

 by simple collection of the emitted rays to a conjugate focus, is 

 confined to the delineation of self-luminous objects, those which radiate 

 by transmitted or reflected light being depicted on the interference 

 principle. 



The explanation of the mode of delineation of coarse structures 

 would then stand thus : — The image of any given structure becomes, 

 as we have seen, more and more similar to the true composition of the 

 object, when a greater and greater portion of the whole diffraction 

 group is admitted to the objective, or the lost portion is reduced more 

 and more. The dissimilarity of the microscopical image in relation 

 to the object depends only on the loss of diffracted rays, and is there- 

 fore in general increased when the objective's aperture excludes more 

 and more of the bent-off beams. The image of a structure which is 



* A striking instance of this diflSculty will be found in the statement wliich 

 recently appeared in print that " these mysterious spectra are simply visionary " 

 (Eng. Mech., xxxii. 1880, p. 300), a statement which was made with so much 

 genuine conviction that it mtcst he true that it was not thought necessary to 

 check the assertion by an experiment which it would not have taken five minutes 

 to make ! And this in an age which has seen sound conveyed to a distance by a 

 beam of light (or heat), and in which no one any longer ventures to discredit 

 in advance a reported new discovery merely by the suggestion that it outrages 

 tlie notions that we were taugl.t in early life, and which we had got to believe 

 were so simple and fundamental that it was impossible they could be either 

 erroneous or capable of qualification. 



