The Abbe Diffraction Theory. By J. W. Gordon. 



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Fig. 71. 



dismounted at the back of the objective, for the purpose of modifying 

 its aperture. A description in detail of the various forms of these 

 diaphragms would serve no useful purpose. So far as they are 

 material to the present discussion, they are figured in Dr. Zimmer- 

 mann's drawings, and will be reproduced here in that connexion. 



(4) Lastly, an objective, a a of the Zeiss series, is recommended 

 for use with the rest of the apparatus, and is supplied for the purpose. 



We are now in a position to follow Dr. Zimmemiann's description. 

 He says (p. 40, § 69) " For the purpose of making a direct observa- 

 tion in the Microscope of these diffraction images,* the so-called Abbe 

 Diffractions Platte may be employed with advantage, upon which are 

 traced three several groups of lines ruled in a thin film of silver. We 

 shnll here limit our attention to the midmost of these groups which 

 contains only parallel lines spaced, in one half of the area occupied, 

 twice as far apart as in the other 

 half." He then gives a drawing, 

 here reproduced as fig. 71, which 

 he says is only simplified by 

 diminishing the number of lines 

 to one-fourth of the actual num- 

 ber. He then continues : — 



" In order to observe the dif- 

 fraction image yielded by this 

 group of lines, the observer first 

 focusses the Microscope provided 

 with a low-power objective (about 

 Zeiss a a or A), and under strong 

 illumination upon the ruling in 

 the usual way, then places on the 

 stop stage of the Abbe substage 

 condenser, or in the ordinary stop 

 cylinder, a stop of blackened cardboard provided with a narrow slit 

 less than \ mm. wide ; and having disposed this so that it lies parallel 

 with the lines in the ruling, he removes the ocular and looks down 

 the tube of the instrument. He will then see a white image of the 

 slit flanked by a large number of diffraction spectra." 



This is represented by another figure, here replaced by fig. 72,| 

 and our author proceeds : — 



" If now he moves his eye above the Microscope in a horizontal 

 direction parallel with the lines in the ruling, he will notice that the 

 number of the spectra is suddenly doubled. This arises from the fact 



* By diffraction images he means the fine "structure" details of a microscopic 

 image. 



f As this and some other of Dr. Zimmermann's figures are elaborately designed 

 drawings, it has been thought more proper to re-draw this and the other figures in 

 question from Nature than to borrow the artistic features of the very beautiful plates 

 which illustrate Dr. Zimmermann's work. 



