ON DIFFRACTION SPECTRA IN THE MICROSCOPE. 29 



interspace be focussed alone in the fields there will be seen on 

 looking into the tube a series of seven bands, of which the centre 

 band is the direct image ; and three on each side of this central band 

 which are diffraction images, whose inner edges are fringed with 

 blue color and their outer edges with red. (PI. II., iig. 3.) On 

 shifting the object so as to get the other set of lines (ruled with 

 closer interspaces) into the field — for which purpose it is best to 

 replace the eyepiece and re-focus the group — and on withdrawing 

 the eyepiece and looking again down the tube three bands only will 

 be seen : a central direct image, and on each side of it a lateral 

 diffraction image. (PI. II. fig. 4.) The space between these bands 

 is just double that between the bands just seen; because the 

 distance between the bands of all diffraction spectra is always 

 inversely proportional to the interspace of the actual lines or 

 pirticles causing diffraction. If, now, the object plate be again 

 shifted, so as to get the two sets of lines seen together in the centre 

 of the field, (pi. II., fig. 2), in one continuous series of parallel 

 lines, just where the set of close ruled lines ceases, and the 

 eyepiece be again withdrawn, (the slits on the card diaphragm also 

 remaining at right angles with the lines,) the diffraction spectra of 

 both sets of lines are seen together ; whereby it may be noticed 

 that the spectra of the wider set of lines exactly cover those of the 

 narrower sets, and also occupy the middle of each wide interspace 

 between the spectra of the narrow lines, with an intermediate 

 diffraction spectrum, (fig. 4.) 



Whilst looking down the tube we now turn the diaphragm 

 round until the three slits take an exactly parallel position with the 

 lines of the object, i.e., the bands seen in the field of view 

 (pi. II., fig. 4.) It will now be found that the centre slit admits 

 the direct image, and the slit on each side of it admits the second 

 diffraction image of the wider lines, together with the ^rst 

 diffraction image of the narrow lines. That is to say, the effect of 

 the diaphragm in its present position is to shut off the first and 

 every other alternate diffraction image of the wide ruled lines. 

 But on replacing the eyepiece and examining the microscope image. 



