0.17 mm, this being the thickness with respect to which the objec- 

 tives have been corrected. 



Independence of the cover-glass thickness may be secured by 

 the use of objectives fitted with correction collars. The apochro- 

 matic objective of 4 mm focus and the achromatic lens No. S, which 

 are both supplied with a correction collar, are to be specially recom- 

 mended for this purpose. It may, in fact, be said generally that 

 apochromatic lenses are much to be preferred in dark-ground illu- 

 mination, since this mode of observation is extremely sensitive to 

 differences of colour. 



The method is also available for observation with the aid of 

 immersion lenses, which offer a two-fold advantage. In the first 

 place, immersion lenses are within wide limits independent of varia- 

 tion in the thickness of the cover-glass, and in the second place, 

 the image of the object is brighter owing to the absence of reflections 

 at the surface of the cover-glass and of th"e-*front lens of the ob- 

 jective. When used in this way the immersion lens should be 

 stopped down sufficiently so that only the diffused rays enter the 

 eye, while the direct rays do not. Despite this limitation, the aper- 

 ture can be made greater than that of a dry lens, which again is 

 an advantage. 



In the case of the Via" and l j\>"a lenses the necessary stopp- 

 ing is obtained by simply screwing a funnel into the objective. 



As in the case of the dry lenses, the apochromatic lenses, 

 e. g. the apochromatic 2 mm or the fluorite 1 n"a oil-immersion 

 lens, yield the finest and brightest images. In the case of the 

 2 mm apochromatic lens the reduction of the aperture has to be 

 applied with much greater care and cannot be effected by the ob- 

 server himself. The necessary modification does not, however, 

 render the objective useless for other purposes, for the stopping 

 devices may be removed by us. 



Having screwed the objective to the microscope tube, slip the 

 dark-ground illuminator into the condenser sleeve, place the pre- 

 paration on the stage, and having placed a drop of oil on the top 

 of the illuminator, raise the illuminator until it touches the slide. 



The light should now by means of a bull's eye lens be 

 directed upon the plane mirror. In the case of the 4 amp. arc lamp 

 listed below the illuminating lens is attached to the lamp casing 

 and the lamp is placed at such a distance from the microscope that 

 the observer may conveniently reach the carbon regulator whilst 



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