350 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Dealing now with the phenomena of diifraction as more immediately 

 connected with microscopical vision, the existence and influence of the 

 diffraction spectra in the Microscope may be demonstrated theoreti- 

 cally and experimentally. The theoretical demonstration has been 

 given by Professor Abbe in his original paper,* and a simple and 

 intelligible exposition will also be found in Naegeli and Schwen- 

 dener's ' Das Mikroskop.' | 



The point which is of importance for our present purpose in 

 Abbe's researches is that Fraunhofer's formula was for the first time 

 applied to microscopical vision, and the influence of the diffracted 

 light on the image investigated. 



If a diaphragm-opening be interposed between the mirror and a 

 plate of ruled lines placed upon the stage, the 

 '^'" ■ appearance shown in Fig. 92 will be observed at 



the back of the objective on removing the eye- 

 piece and looking down the tube of the Micro- 

 scope. The central circle is an image of the 

 diaphragm-opening produced by the direct, so- 

 called non-diffracted, rays, while those on either 

 side are the diffraction images produced by the 

 rays which are bent off from the incident pencil. 

 In homogeneous light the central and lateral 

 images agree in size and form, but in white light, as might have been 

 expected, the diffracted images are radially drawn out, with the outer 

 edges red and the inner blue (the reverse of the ordinary spectrum), 

 forming, in fact, regular spectra, the distance separating each of which 

 varies inversely as the closeness of the lines, being, for instance, with 

 the same objective twice as far ajiart when the lines are twice as 

 close. 



The formation of the microscopical image is explained by the 

 fact that the rays collected at the back of the objective, depicting 

 there the direct and spectral images of the source of light, reach in 

 their further course the plane which is conjugate to the object and 

 give rise there to an interference phenomenon (owing to the con- 

 nections of the undulations), this interference effect giving the 

 ultimate image which is observed by the eye-piece, and which there- 

 fore depends essentially on the number and distribution of the dif- 

 fracted beams which enter the objective. 



That the diffraction spectra are not mere superfluous and acci- 

 dental phenomena but are in fact directly connected with the image 

 seen by the eye, has been very fully demonstrated by Professor Abbe 

 at a special meeting held for the purpose, and recorded in our Pro- 

 ceedings in the Journal, as well as in papers by Mr. Stephenson 

 before this Society, by Dr. Fripp before the Bristol Naturalists' 

 Society, and by ourselves before the Quekett Microscopical Club. 

 These papers are printed at full length in the Proceedings of the 

 respective Societies, with the illustrations used, so that we may 



* See references collected in this Journal, ii. (1879) p. 651. 

 t 2n(l ed. (1877), English Translation (1881), p. 233. 



