60 PEACTICAL PHOTO-MICKOGEAPHY 



a bright image on a dark ground ; but it is not so easily 

 applicable to ordinary methods of observation as those previously 

 described, and entails a special arrangement above the ocular 

 for supporting the stop. It is also necessary that the stop 

 itself should be very small and exactly suited in size to the 

 conditions of observation. As these two latter points are of 

 considerable practical difficulty, the method has not reached 

 a stage where it can be regarded as generally applicable. 



In the case of diatoms, or similar objects, it is often necessary 

 for the illumination to be in one azimuth only, and for this 

 purpose stops are designed which allow of the utilisation of 

 certain portions only of the cone of light passing into the 

 objective, the remainder being stopped out as required. As 

 this method is of rather special application, further reference 

 to it will be made when dealing with the methods of photo- 

 graphing particular objects. 



It must be clearly understood that dark-ground methods 

 do not result in any increase of resolution, although they may 

 render visible isolated particles or the components of irregular 

 structure that are not easily seen in other ways. 



The resolving limit may be found by multiplying the 

 N.A. of a given objective by 80,000 when monochromatic green 

 light is used, and illumination is by a solid axial cone of light. 

 With oblique illumination in one azimuth and with a small 

 portion only of the peripheral zone in use, the power of 

 resolving regular structure is increased by about one-third ; 

 but this latter method is practically only applicable to the 

 illumination of such objects as diatoms or similar ones in which 

 there are considerable diffraction effects in the objects them- 

 selves. The limit of visibility, so far as isolated objects are 

 concerned, is quite a different point, and Lord Kayleigh states 

 that isolated objects, or two bright areas separated by a dark 

 line, may be seen if the dark line is as narrow as one-sixteenth 

 and under certain conditions one thirty-second of the wave- 

 length of the light used, although the resulting image does not 

 of necessity represent the actual appearance of the object. The 

 great factor under such circumstances is the intensity of the 

 illuminant, and there is much reason to conclude that any 

 isolated object may be rendered visible, even if it approaches 

 molecular dimensions, if it is able to reflect sufficient light for 



