249 



In familiar language, diffraction was the power which light possessed of 

 going round a corner under certain conditions. (This was illustrated by a 

 sketch on the black board, showing that solid opaque substances had the 

 power of bending light, a finely ruled or granular surface also doing the 

 same.) A very familiar example' could be seen at night in any railway car- 

 riage through the steamy or smeared windows, where curious irradiations 

 surrounded the distant gas lights as seen through the dirty glass. 



Now when a microscopic object possessed structural details exceeding 

 a certain degree of fineness, it acted precisely as a ruled diffraction grating, 

 and gave rise to a series of coloured images (diffraction spectra), varying 

 in number and arrangement according to the nature of the structure. These 

 surrounded the before-mentioned pencil of light (Dioptric beam), and 

 joined with it in producing the microscopic image, the function of the 

 diojJtric beam being confined to the rendering of general contour with a 

 very limited amount of detail, while the diffraction spectra, according to 

 Professor Abbe, alone resolved detail exceeding a certain degree of minute- 

 ness. 



The Professor had devised a few simple experiments to prove the truth of 

 his theory, which, although easily exhibited to one or two persons, were 

 extremely difficult to demonstrate to a large number. He (the speaker) 

 had, however, prepared a series of models or diagrams which would convey 

 a tolerably clear notion of the experiments to those present, and he would 

 afterwards endeavour, with the assistance of Mr Ingpen, to show the 

 various effects with the instruments on the table. 



The diagrams illustrating the formation of the spectra, and the shape and 

 position of the slits in the diaphragms by which the spectra were examined, 

 were shown on a scale sufficiently large for those present to see, the various 

 discs being slipped over a blackened background, so that they could be 

 easily revolved and changed as required. [The plate illustrating Mr. 

 Crisp's paper on Diffraction (Plate VII. , Vol. v. of the Club Journal), and 

 which is fully explained in Mr. Crisp's paper (see p. 79 et seq. of the same 

 volume), will enable the reader to follow the experiments here described.] 



Professor Abbe's experiments were made on a known object, with an ob- 

 jective of low power, and under conditions which were entirely under the 

 control of the operator. The objective was Zeiss's " aa, 32 mm. nominal 

 focal length" (about 1£ inches English) ; numerical aperture 0'17 = 20° 

 angular aperture. The object was a series of two sets of fine lines ruled on 

 silvered glass, the lower set being twice as close as the upper set. As the 

 dividing engine was employed in the ruling process, the distance of the 

 lines and the exact nature of the object was placed beyond doubt. 



The next step was to find some means of examining the emerging 

 pencils at the back of the object-glass. This could be done by removing 

 the eye-piece and looking down the tube, but the most effectual way was 

 that suggested by Mr. Ingpen, the employment of a small Ramsden eye- 

 piece placed above the ordinary ocular, the whole combination forming a 

 email telescope, which gives a clear view of what is taking place at the back 

 of the object-glass. 



