310 The Outline of Science 



that the busy bee was not the chief agent in its production. In 

 short, the microscope is a valuable detective of dishonesty. But a 

 use of the microscope more important and more pleasant to think 

 of is in metallurgy, where its utilisation to detect the structural 

 features of the stable and the transient in various metallurgical 

 combinations, such as different kinds of steel, has been of in- 

 estimable importance. 



A farmer can always make good use of a lens in examining 

 samples of seeds, or in identifying particular kinds of injurious 

 insects, or in detecting the beginnings of "rusts" and "mildews" 

 on his crops. But the expert agriculturist must of course go much 

 further, especially in warm countries, where the microscope is 

 necessary for the study of the insidious Fungi which are always 

 ready to find a weak spot in the plants' defences in all sorts of 

 plantations from coffee to rubber. 



The Ultra-Microscope 



Early in the twentieth century an ingenious method was de- 

 scribed by Siedentopf and Zsigmondy, which is often briefly re- 

 ferred to as the ultra-microscope. Everyone knows from personal 

 observation that a strong beam of sunlight entering a darkened 

 room reveals a multitude of dust particles, which are not seen at 

 all in ordinary light. The same multitude of particles is often 

 seen in the track of a strong beam from a "magic lantern" in 

 a darkened room. These dancing particles, whose abundance we 

 scarcely suspected, become visible because they are so strongly 

 illumined; there is a diffraction of rays from their surface, and 

 they look much bigger than they really are. 



In 1899 Lord Rayleigh pointed out that a particle too small 

 to be seen by the highest power of the microscope under ordinary 

 conditions might be made visible if it received sufficiently intense 

 illumination; and the ultra-microscope took advantage of this 

 idea. It occurred to Siedentopf and Zsigmondy that if the par- 

 ticles in a solution could be strongly illumined by a beam coming 



