A NEW MICROCOSM 5 



Abbe has calculated that the utmost attainable 

 limit of resolving power can never exceed TTTTOTF in -> 

 on account of diffraction. This means that when 

 objects are any smaller than that, and are viewed 

 by light transmitted along the axis of the micro- 

 scope, the light bends round the object, and enters 

 the eye just as if the object did not exist. Hence 

 the object is invisible. This difficulty could be 

 overcome if photographs could be obtained by 

 extreme ultra-violet light, which, on account of 

 the shortness of its constituent waves, is less bent 

 aside from its rectilinear course. 



But diffraction is not an unmixed curse of optics. 

 It may be utilised for ultra-microscopic vision. Of 

 this there are two familiar examples. The fixed 

 stars show no measurable diameter, even with 

 the highest magnifying power. They would be 

 quite invisible but for the fact that a parallel 

 beam of light, incident upon the eye or upon any 

 other achromatic instrument, is not brought to a 

 focus in a geometrical point, but in a small 

 produced by diffraction, a disc sufficiently large 

 to come within the perceptive power of the 

 Oft, 



The motes floating in a sunbeam, again, would 



be quite invisible but for diffractin. They are 



less than unnnnr i- in diameter, as can be proved 



by their rate of settling down in still air. Their 



v smollnesa enables them to scatter the light 



