THE MICROSCOPE AND ITS ACCESSORIES. 



II 



bacteria with objectives of large aperture. In every other case A 

 diaphragm of suitable size is introduced so as to diminish the 

 cone of light. When the diaphragm is placed excentrically, oblique 

 illumination is obtained, while, 

 with a central-stop diaphragm 

 all the axial rays are cut off, 

 and dark-ground illumination is 

 obtained. 



11. Dry and Immersion 

 Lenses. By the term dry lens 

 is meant one in which air is 

 the medium between the lens 

 and the object, or at least the 

 cover-glass on the object. In 

 immersion lenses some fluid 

 intervenes between the lowest 

 lens of the objective and the 

 cover-glass of the object, and 

 the liquid chosen is water, or a 

 medium of higher refractive 

 index, such as cedar-wood oil, 

 glycerine, or a mixture of 

 fennel and ricinus oils. These 

 oils have nearly the same re- 

 fractive index as the cover- 

 glass. In virtue of their greater refractive power, these liquids, 

 especially the oils, refract more of the rays passing through the 

 object, and cause these rays of light to enter the lens, so that they 

 increase the amount of light transmitted to the eye of the observer. 

 In contrast with dry lenses, they have a larger angle of aperture 

 and a greater resolving power, and are employed only for the 

 highest magnifying powers. Powers above \ of an inch should be 

 oil-immersion lenses. Oil-immersion or homogeneous immersion 

 lenses are to be preferred to water ones. The oil-immersion are 

 fast displacing water-immersion lenses. 



In using an oil-immersion lens, by means of a glass rod, place a 

 small drop of thick cedar-oil on the cover-glass and a small drop on 

 the lowest lens of the objective, and slowly depress the objective 

 until it touches the cupola of the drop, and focus. Cedar-oil has a 

 refractive index nearly the same as that of crown-glass, so that 

 almost all the rays of light passing through the object reach the lens 

 and pass up the tube of the microscope. Care must be taken that 

 the drop of oil docs not run on to the cement of the preparation, 

 else it will dissolve it. It is better to use marine glue to seal up 

 the preparation, as it is not dissolved by cedar-oil. A very fine 



FIG. 13. Abbe's Condenser, as made by Zeiss 

 and Swift. (S.) condenser system. Milled 

 bead for throwing the diaphragm out of 

 centre ; (Sp.) mirror. 



