CH. VIII} 



PHO TO-MICROGRAPHY 



227 



objects a Steinheil lens combination gives a more brilliant light and one also more 

 nearly achromatic. For high power work all are agreed that nothing will take the 

 place of an achromatic condenser. This may be simply an achromatic condenser 

 but preferably it should be an apochromatic condenser. Whatever the form of the 

 condenser it should possess diaphragms so that the aperture of the condenser may 

 be varied depending upon the aperture of the objective. For a long time objec- 

 tives have been used as achromatic condensers, and they are very satisfactory, 

 although less convenient than a special condenser whose aperture is great enough 

 for the highest powers and capable of being reduced by means of diaphragms to the 

 capacitv of the lower objectives. It should also be capable of accurate centering 

 (§81). 



Fig. 186. Arrangement for Artificial Illumination. 



1. Lamp with metal chimney, easily made by rolling up some ferrotype plate 

 and making a slit-like opening in one side. This opening should be covered by an 

 oblong cover-glass. A glass slide, being of considerable thickness, breaks too easily. 

 The lamp should have a wick about 40 mm. wide, so that the thickness of the flame , 

 if taken edgewise, will give an intense light. A wide flame also enables one to get 

 a larger image of the flame, and thus to illuminate a larger object than as though a 

 small flame was used. 



2. Bull's-eye condenser on a separate stand. The engraving glass shown in 

 Fig. 188, or the tripod magnifier {Fig. 172) answers fairly . The Steinheil lenses 

 are still better. 



3. Screen showing image of the flame inverted. 



The lamp and bulls-eye stand are on blocks with screw-eyes as leveling screws. 



I 368. Objects Suitable for Photo-micrographs. — While almost any large ob- 

 ject may be photographed well with the ordinary camera and photographic objec- 

 tive, only a small part of the objects mounted for microscopic study can be photo- 

 micrographed satisfactorily. Many objects that give beautiful images when look- 

 ing into the microscope and constantly focusing with the fine adjustment, appear 

 almost without detail on the screen of the photo-micrographic camera and in the 

 photo-micrograph. 



