PRACTICAL PHOTO-MICROGRAPHY. 1Y5 



parallelize the rays on the back of the condenser. Now one 

 or other of the black discs is placed in the condenser, which 

 may be racked up and down so that the best image of the 

 object and the blackest ground possible are attained. The 

 point of chief importance is to get the margins of the object 

 perfectly sharp against the black ground, and the secret of suc- 

 cess is to use the smallest disc that will give a brilliantly-lighted 

 object with an absolutely black ground. The " spot lens " may 

 aptly be likened to the above arrangement minus the power of 

 altering the size of the spot, and the parabolic illuminator 

 may take a place midway between the two, for it has a certain 

 range of adaptation in virtue of a small movable "spot" 

 worked from its lower end. We need hardly say that in this 

 work a prolonged exposure is required, more prolonged than 

 might be expected by the beginner. 



FIG. 39. LIEBERKUHN. FIG. 40. WENHAM'S PARABOLIC 



(BECK.) ILLUMINATOR. 



On the whole, we should prefer a spot lens to a paraboloid, 

 were we compelled to use one or other. 



When an opaque mount has to be photographed we should 

 use the Lieberkuhn (Fig. 39) when it is available. With the 

 Lieberkuhn the light passes from the radiant round the object 

 to the Lieberkuhn (which is fixed to the objective) and thence 

 reflected back upon the object, which is thereby illuminated 

 evenly from all sides. This necessitates an object mounted on 

 a black disc having a clear space all round it. 



As the province of the Lieberkuhn is to reflect and focus 

 the rays upon the object, it will be easily understood that the 

 Lieberkuhn has to be made to correspond with a lens of a par- 



