The Abbe Diffraction Theory. By J. W. Gordon. 369 



an alternative to embark upon certain researches by which the de- 

 pendence of the microscopic image upon the diffraction image of the 

 source of light may be made the subject of ocular demonstration. We 

 cannot, however, part from his too brief notice of the theory without 

 remarking that there is no need to be modest in the demand for 

 diffraction spectra. Contentment with a single spectrum strikes one 

 as being a weak reticence, it being so easy, as we have seen, to 

 obtain diffraction spectra in unlimited number in the most limited 

 angular space. But perhaps this comment savours of insubordination, 

 and for the present it is my purpose to accept without reserve all my 

 guide's conditions. 



With this exordium, then, our author comes to the first experi- 

 ment, and says : " We employ again for this purpose the midmost 

 field of the Abbe diffraction plate. We have already seen that two 

 systems of ruled lines are traced upon this field, the one system 



Fig. 73. Fig. 74. 



having the common distance of its linos apart double as great as that 

 of the other system, and that, in consequence, in one half of the 

 diffraction image the spectra stand double as far apart as in the 

 other half. 



" If now a diaphragm* be introduced in the plane of the diffraction 

 images which will allow only the direct image of the source of light 

 to pass (A of fig. 72), and the microscopic image is observed with 

 the replaced ocular, there will in fact be seen, instead of the parallel 

 lines, an almost uniformly bright surface (tig. 74). 



" If, however, the aperture in the diaphragm be made so large 

 that, in addition to the direct image of the source of light, the two 

 nearest diffraction spectra corresponding to the wider system of ruled 

 lines be allowed to pass (fig. 75), f then, in fact, the individual lines of 



* The diaphragm referred to is shown in fig. 73. 



t This diaphragm allows the group of spectra — including|in the term spectra the 

 central image —to pass, which is marked B in fig. 72. 



Aug. 21st, 1901 2 c 



