CHAPTER XXYI. 



BLACK BACKGROUNDS. OPAQUE MOUNTS. 

 POLARIZED OBJECTS. 



PHOTOGRAPHY of certain lustrous objects upon a dark ground 

 is not merely, as some think, a playful way of producing a 

 sensational picture, but is in many cases a really useful method 

 of depicting suitable objects. We have seen a lecture on 

 Diatom Structure illustrated almost entirely by lantern-slides 

 of diatoms on black ground ; many crystals, and certain eyes 

 are better seen on such a ground than on a white one. 



When objects are "mounted opaque" they require to be 

 photographed by reflected light, and the background is then 

 naturally dark, but when objects are mounted in the common 

 way on clear glass, we require for a black ground one or other 

 of several optical instruments. First we mention a " spot lens," 

 which, before it became obsolete, was used below the stage, 

 but is now entirely replaced by a "paraboloid," or, better, by 

 the addition of certain disc-stops to an ordinary substage con- 

 denser. The paraboloid is a very pretty piece of optical 

 ingenuity, sometimes called "parabolic illuminator," but in 

 practice the condenser with stops will be found superior. 



For this class of work the achromatic substage condenser is 

 usually furnished with a set of the discs figured No. 5, and 

 there is in the fittings of the condenser a slot or other recep- 

 tacle to receive the stops. With powers higher than a quarter- 

 inch it will be found somewhat difficult to work the system 

 properly, still it is not impossible to obtain good results even 

 with immersion glasses. The practice is somewhat as follows : 

 The condenser is centred, the condenser and objective focused 

 as usual on the object, but in the present work a bull's eye 

 may without detriment, and even with advantage, be used to ~ 



