54 PRACTICAL PHOTO-MICROGRAPHY 



no pretensions to be regarded as an accurate method of illu- 

 mination. It consists merely of a condensing-lens, between 

 which and the mirror of the microscope a black stop is intro- 

 duced ; the result is that any object placed in the position of 

 the focal point of the lens is illuminated only by oblique rays 

 the central direct rays being obstructed. The proportion of the 

 total light condensed on to the object which the central spot 

 cuts out must be greater than the aperture of the objective 

 itself. Hence, on the assumption that the object is mounted in 

 a medium having a refractive index substantially different to its 

 own, any object placed at the apex of this illuminating cone 

 shows up as a bright object on a dark ground. 



Two other methods for effecting the same purpose are the 

 use of a paraboloid, either dry or oil-immersion, or the intro- 

 duction of a black central stop into or behind the ordinary sub- 

 stage condenser. The paraboloid, unless it is one of the modern 

 type which has recently been introduced by Messrs. Zeiss, has 

 become practically obsolete ; but the method of using a stop in 

 the sub-stage condenser still remains one of the best with 

 low- or medium -power objectives. The advantage of the latter 

 is that the stop may be varied at will, and can be suited to the 

 objective in use. 



A contrivance of recent introduction allows the size of 

 the central stop to be varied much in the same way that 

 the size of the opening of an iris -diaphragm can be varied. A 

 series of leaves are arranged at the centre and underneath the 

 sub-stage so that they can be opened or closed at will ; the 

 size of the stop can thereby be altered to meet varying 

 conditions, or to suit any change of objective. 



It is obvious that the method of using the sub-stage con- 

 denser is only serviceable where the condenser has a numerical 

 aperture substantially greater than the objective, as the 

 illumination depends largely on the excess aperture of the 

 condenser. Unless the excess is sufficient to transmit light 

 enough to fully illuminate the object, and for it to be easily 

 appreciated visually, then the method is unsatisfactory. 

 This condition does not obtain with paraboloids of the older 

 type, since to get the best results the latter should be suited 

 to the objective. The stop used should in any case be of 

 such a size that it obscures more than the central two-thirds 



