30 ON DIFFRACTION SPECTRA IN THE MICROSCOPE. 



we now find the whole visible group of lines no longer exhibiting, 

 as before, a field half occupied by widely ruled lines and half by 

 narrow ruled lines, but entirely occupied by the narrow system of 

 lines, (pi. II., fig. S-) '^^^ wider lines have doubled in number, 

 the narrower remaining unaltered : and the only difference between' 

 the direct and diffraction images is, that the lines appear brighter 

 where the images of the wide and narrow systems coincide ; but 

 less bright where the doubling of the wide system occasions lines 

 to be seen in interspaces, the light of which does not come from as 

 many actual lines in the object as appear in the field. 



The three slits in the diaphragm used for the above experiment 

 are separated from each other by an interspace equal to their own 

 width; but if separated by an interspace of treble their width' 

 (pi. II., fig. r 5.), then the second spectrum of the narrow system 

 of lines, and all but the fourth of the wider system, are shut off, 

 (pi. II., fig. 4,) with the effect of doubling the number of lines 

 now seen in the microscope image, (fig. 5), (for which a deep 

 eyepiece is needed to amplify the image.) As both systems of 

 lines are affected, the narrow system is doubled and the wide 

 system quadrupled. 



Thus it is proved by this experiment that diffraction images are 

 produced by a certain class of objects outside the microscope ; and 

 that the microscopic image, of an object causing such diffraction, 

 will vary with the treatment of the diffraction spectra, so that no 

 positive conclusion regarding its actual structure (if an unknown 

 object) can be drawn from the visible lineation solely. This will 

 appear still more strongly from 



Experiment II. — 



The same object is used, but the card diaphragm is changed for 

 one having a single slit instead of three. Taking the same 

 precautions as before, the instrument is focussed on the object, 

 which may be shifted so as to get the groups of wide and narrow 

 lines separately in the field. The diaphragm i, c, with narrow slit 

 must be employed when the group of wide lines is viewed through 



