RESOLUTION 151 



or by throwing the condenser out of focus ; the illumina- 

 tion then becomes " non-critical." For fine work, if the 

 illumination is too intense, this should be diminished by 

 diminishing the source of light or by interposing a screen. 

 These points are dealt with below under " Illumination." 

 The lenses or objectives are either dry or immersion ; 

 the dry lens works with a layer of air between it and 

 the object or cover-glass covering the object, the immer- 

 sion lens has a layer of fluid water in the water 

 immersion, and cedar-wood oil in the oil-immersion, 

 forms between it and the object. Whereas magnification 

 depends upon the focal length of the objective the 

 smaller the focal length, the greater the magnification 

 and a dry lens can be constructed having as great a 

 magnification as any immersion one, resolution, that is, 

 the capacity for seeing minute objects or structure, 

 depends upon another factor which is summed up in the 

 expression " numerical aperture " ; the greater the 

 numerical aperture of the lens, the greater its resolving 

 power. For a dry lens the theoretical numerical aperture 

 cannot exceed 1-0, for a water-immersion lens 1-33, and for 

 an oil-immersion lens 1*515. The geometrical theory of the 

 telescope and photographic camera is based upon the 

 rectilineal propagation of light, but when we employ the 

 microscope we may ultimately deal with objects which 

 are in size commensurable with the wave-length of light 

 (say, 4 y*- in. or 0-51 //,), and they have a scattering effect 

 on the light beam passing through or about them. Light 

 thus scattered by small objects (or apertures) is known as 

 diffracted light. Now, resolution depends upon the capa- 

 city of the objective to collect and pass through it this 

 scattered or diffracted light, and this depends for the 

 most part upon an increase in the ratio of the semi- 

 diameter of the back lens to the focal length of the system 

 of lenses of the objective. The more the objective can 



