DARK-GROUND ILLUMINATION 163 



of the condenser, as in the Leitz spherical surface-reflecting 

 condenser. In the Zeiss paraboloid the marginal parallel 

 rays of light strike the lateral surface of the condenser, 

 which is ground to a paraboloid form, and are thence 

 reflected so as to come to a focus on the object on the 

 slide. In the Leitz reflecting condenser a hemispherical 

 cavity is present in the centre of the condenser, and both 

 the central and marginal rays striking the hemispherical 

 surface are reflected to the side of the condenser, whence 

 again they are reflected upwards, so as to be brought 

 to a focus on the object on the slide. 



It is obvious from this brief description of the prin- 

 ciples involved that the perfection of the dark -ground 

 condenser depends upon the perfection with which the 

 rays are brought to a focus upon the object or objects, 

 and this will depend partly upon perfection in construc- 

 tion and partly upon proper adjustment of the light by 

 the user. It is also obvious that to obtain exact focussing 

 of the light upon the objects the slide must be within 

 certain limits of thickness ; it must not be too thick, 

 for the light cannot then be focussed on the object ; if 

 on the thin side, an increased amount of oil, so that 

 the condenser can be racked down, will generally 

 suffice. 



Illumination is all-important for perfect dark-ground 

 work. The illuminant should be small in area and fairly 

 intense, but it is a mistake to have it too intense if it be 

 desired to see fine details. In the past, and to a consider- 

 able extent now, a small arc lamp has been used, but it 

 errs on the side of intensity. The lamp should run with 

 about 4 amperes, so that it may be used on the ordinary 

 lighting circuit. The carbons should be mounted at right 

 angles to one another, the horizontal carbon being con- 

 nected with the positive lead on the main, and a hand 

 feed is provided to maintain the proper distance of the 



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