Notes. 



569 



in the back focal plane of the objective. The image of the prating results 

 from the reunion of light coming from these spectra, and, according to the 

 well-known law of Abbe, no denned appearance of structure can be seen in 

 the image, unless at least two of these spectra have taken part in its formation 



The aim in view was to see whether, after blocking out all but one of the 

 spectra diffracted by the object, it was possible to produce precisely similar 

 spectra in some other way to replace them, and, if so, what would happen to 

 the image. 



An Abbe Demonstration Microscope was used by the author for the in- 



vestigation. In this instrument Abbe's view that a Microscope objective is 

 equivalent to a magnifying lens J.T (fig. 133), to convert the divergent rays 

 from the object into parallel pencils, and a telescope objective T T to bring 

 these parallel pencils to a focus and produce the image, is embodied in a 

 practical form. 



The spectra V G K and V 1 G 1 K 1 (fig. 134), diffracted by the object, were 

 blocked out by means of the diaphragm D D,' so that only the central beam (or 

 zero spectrum) was transmitted, and in order to imitate these spectra the 

 device was hit upon to pass the beam through two diffraction gratings of the 

 same pitch. Thorp gratings of about 14,500 lines per inch were used, and 

 are shown at F F l and H H' (fig. 134). 



To consider the action of passing parallel light through two gratings of 



