370 LIFE : OUTLINES OF GENERAL BIOLOGY 



best seen when the difficult method of "dark-ground illumination" 

 is employed, in which particles too small to be seen even with the 

 strongest lenses betray their presence by intercepting rays of light 

 and appear as bright sjx)ts in a dark field. Further details are beyond 

 our scope at present, but the interesting general fact is that the 

 bright spots are seen to move freely through the protoplasm, which 

 would not be possible if a solid reticulum were present. 



The use of ultra-violet rays instead of visible light for micro- 

 scopic photographs has also been helpful, for in this region of the 

 spectrum of radiations some materials quite transparent to ordinary 

 light appear opaque, and the detailed structure of cells is thus more 

 easily discerned. Unfortunately, however, ultra-\iolet hght has a 

 disturbing action on fluid protoplasm. 



In ordinary microscopic observation, whether with visible or with 

 ultra-vdolet light, the object examined absorbs some of the trans- 

 mitted light reflected through it from the mirror, and the structure 

 of the object appears relatively opaque or coloured. In the "dark- 

 ground" method, no light reaches the eye at all except in so far as 

 rays are refracted or scattered by particles in the object itself. The 

 illumination of the object is in a horizontal, not in a vertical 

 direction. 



No amount of mere magnification will make it possible to sti' 

 definitely a particle with a diameter less than one five-thousandth 

 part of a millimetre. It is too small for the light-rays of the wave- 

 lengths visible to our eyes; "resolution" cannot be obtained with 

 any system of lenses. No object smaller than half the wave-length 

 of the light used can be seen with any definiteness. This is a matter 

 of considerable practical importance in the study of the "filterable 

 viruses" which are believed to contain organisms smaller than 

 bacteria that are associated with such diseases as hydrophobia, 

 measles, foot-and-mouth disease, and cancer. 



But when the object is altogether below the limit of resolution 

 by ordinary microscopy, it is still possible to detect its presence by 

 an extension of the "dark-ground" method — the so-called "ultra- 

 microscope", an instrument first used by Siedentopf and Zsigmondy 

 in 1903. Everyone is familiar with the fact that dust particles, which 

 are absolutely invisible in the ordinary daylight of a room, may 

 be detected in thousands when a strong beam of light suddenly 

 enters. In actual fact the invisible does not become visible, for 

 what we see are the rays diffracted from the surface of the bril- 

 liantly illuminated particles. Similarly, the strong horizontal beam 

 used in the ultra-microscope reveals shining points of diffracted 

 light from minute particles in a fluid. The more intense the illu- 

 mination, the greater the possibiHty of detecting minute particles, 

 and this goes as far as the starch molecules in a solution! It must 

 be emphasised, however, that the particle detected is only indirectly 



