628 



Transactions of the Society. 



ing of a limited number of dark lines in an otherwise bright field. 

 This want of focus accounts for the appearance often noticed by 

 microscopists (and occasionally immortalised by photography !) of 

 striated diatoms or similar objects, shown by a narrow pencil of 

 oblique light, with the detail partly outside the outline of the 

 object. 



This want of focus is, however, never seen in practice to the 

 extent which theoretical considerations prove to be possible, for 

 the simple reason that we do not and cannot realise the condition 

 of monochromatic light from a single distant point. Even when 

 we use a source small enough to approximate a point, it will 

 yield light of various colours ; and as soon as we consider, say, 

 white light, we find a substantial improvement. For we shall then 

 have a diffraction spectrum instead of a single diffracted wave, and 

 consequently there will be, instead of the single point F in fig. 97 



Fig. 9:». 



and fig. 98, a continuous row of points of different colours, each of 

 which co-operates with the waves of the same colour expanding 

 from the focus P of the direct light. Consider two extreme colours, 

 and let P R and P v (fig. 99) be the foci of the red and violet diffracted 

 waves. The lines along which the images of the grating slits 

 appear will aim at the bisecting point C E for the red, at the 

 bisecting point C 7 for the violet light ; and if we draw these lines 

 of maximum brightness, and remember that those corresponding 

 to intermediate colours will fill the space between the extreme 

 ones, we immediately see that there will be confusion at a little 

 distance in either direction from the plane 1 1 of the true image. 

 The images of the slits will be drawn out into spectra which over- 

 lap more and more, and eventually fuse into uniform whiteness. 

 Therefore we shall have a much better criterion for finding the 

 true image, and a much reduced range of focus within which we 

 mi^ht be in doubt. 



