370 



Transactions of the Society. 



the wider ruling will appear, although proportionately much thicker 

 than in reality (fig. 76) ; but in place of the narrow ruling there is still 



Fig. 75. 



Fig. 76. 



only an uniformly bright surface to be seen. Here then a line system 

 corresponding to the actual object comes first to view only when at 

 least one of the diffraction spectra derived from it passes into the 

 Microscope, as is shown in figs. 77* and 78. 



" Finally, the following experiment is very instructive, in which a 

 diaphragm provided with three narrow openings is employed (fig. 79). 

 These slits are so arranged that the midmost of them transmits the 

 direct image, whilst the other two allow the first spectra of finer struc- 

 ture and the second of the coarser to pass ; f the first spectra originat- 

 ing in the coarser structure being cut off by the interposed diaphragm. 



Pig. 77. 



Fig. 78. 



In this case we observe in fact, entirely in accordance with the theory, 

 that in both halves of the image the same number of lines appears. 



* This diaphragm allows the group C of fig. 72 to pass, 

 t That is, the group marked D in fig. 72. 



