168 



PEACTICAL PHOTO-MICKOGBAPHY 



B 



object therefore would not be critically illuminated by a con- 

 denser having a small aplanatic cone if a stop with an annular 

 opening were inserted ; and assuming that the object had been 

 previously illuminated with a solid cone some further adjustment 

 would be required. In such cases a condenser of the highest 

 possible order is essential : it must be well achromatised, and be 

 as free as possible from spherical aberration. 

 fi\ The shapes of the various stops that may be 



used have been already indicated, but in general 

 it will be found that it is more convenient to 

 improvise stops from thin black cardboard, 

 cutting these to the shape and size required 

 to bring out the details of a particular object. 

 For work of the most critical nature, some 

 workers are of the opinion that any auxiliary 

 condensing-lens between the illuminant and 

 sub-stage condenser is to be avoided if possible. 

 In such cases it may be better to use an illu- 

 minant of somewhat greater area, to place it 

 at its proper distance from the sub-stage con- 

 denser usually a distance of approximately 

 ten inches and to focus the image of the 

 radiant without any intervening optical 

 system. The next best arrangement is to 

 use an achromatic auxiliary collecting-lens and 

 to project a parallel beam from a very small 

 source of light, such as the electric arc, into 

 the back combination of the sub-stage con- 

 denser. The writer has yet to find an object 

 that cannot be perfectly illuminated by this 

 method. Among high-power illuminants for highly critical 

 work, lime-light has the advantage that it admits of the 

 direct projection of the image of the radiant in the object- 

 plane without any intervening optical system, since it is 

 both sufficiently powerful and of large enough area. The 

 same may be said of the mercury- vapour lamp, although its 

 intrinsic brilliancy does not approach that of an efficient lime- 

 light jet. 



The Auxiliary Collecting-lens. The advisability of the use 

 of such a lens or lenses is a somewhat controversial point, and 



FIG. 60. Diagram 

 of Path of Rays 

 in Sub - stage 

 Condenser. 



