22 



J. RHEINBERG ON AN OVERLOOKED POINT CONCERNING 



increased or decreased in the direction at right angles to the 

 lines. Immediately in front of the lamp flame there is a screen, 

 with a slot in it of such width that, when the plane of the 

 screen is focnssed by the condenser on to the object in the usual 

 manner, the slot just covers the width of the band of lines 

 under observation. A disc with a very narrow slot in its centre 

 is placed in the stop carrier of the condenser. 



If now, whilst attention is fixed on that portion of the band 

 where the whole of the twelve lines lie next to one another, the 

 N.A. of the objective is gradually cut down to that point where 



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the band of twelve lines is just no longer visible (the lines being 

 blurred into one another), and we then glance down at the 

 position of the two projecting lines, we shall see that these still 

 distinctly appear as separate lines — somewhat thickened and 

 without sharp edges, but yet clearly resolved. 



And what to many may seem remarkable is that, if we remove 

 the eyepiece of the microscope and look down the tube, we see the 

 direct or dioptric beam shining brightly over the centre of the 

 objective, whereas apparently there are no flanking spectra on 

 each side. In looking down the tube in this way, it is necessary 

 to keep the eye perfectly central and without any shift. A 

 simple way to do this is to hold a metal disc with a pinhole at its 

 centre over the tube. The dummy eyepiece on the table is fitted 

 with a metal disc like this. 



.Now for the explanation of the phenomenon. You all know 

 that the condition for resolution of lines is that the angle of the 

 objective must be sufficiently wide to grasp, besides the direct or 

 dioptric beam of light, at least one of the beams diffracted by 



