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will refract a certain number of rays, so that these cannot enter the 

 object-glass; hence certain parts will become dark, and will mapout, as 

 it were, in the image formed of the object, the structural peculiarities 

 of the same. But if the parts are minute, of a curved form and ap- 

 proximatively symmetrical, they will act upon the light transmitted 

 through them in the manner of lenses, and their luminous or dark 

 appearance will vary according to the relation of the foci of these to 

 that of the object-glass. Thus the parts of an object may appear 

 dark and defined, from the refraction of the light out of the field of 

 the microscope ; also, from the concentration or dispersion of por- 

 tions of the light by these parts, all the rays being admitted by the 

 object-glass, or entering the field. 



Another condition affecting distinctness consists in the relation 

 which the luminousness or darkness of an object bears to that of the 

 field or back ground upon which it is apparently situated. 



The refraction of the light out of the field of the microscope or 

 beyond the angle of aperture of the object-glass is the ordinary 

 cause of the outlines of objects becoming visible ; and in these cases, 

 an increase of the angular aperture of the object-glass will impair 

 their distinctness, because it will allow of the admission of those 

 rays which would otherwise have been refracted from the field, and 

 the margins will become more luminous and less contrasted with 

 the luminous field. 



The cause of the distinctness of an object by refraction when 

 all or nearly all the rays enter the field of the microscope, may 

 be investigated in a drop of oil immersed in water, or in a drop 

 of milk, as illuminated by light reflected from an ordinary mirror. 

 The refractive power of the globules is so great and their form 

 such, that each acts as a minute spherical lens ; and the parts 

 within the margin will appear light or dark according to the relation 

 of the focus of the little lens to that of the object-glass. Under an 

 object-glass of small aperture and moderate power the outline will 

 appear black, because the marginal rays do not enter the object-glass. 

 If the object-glass be of sufficient aperture to admit these marginal 

 rays, the black margin will disappear, and the little lens will only be 

 distinguishable by the above focal relation. Its appearance under 

 oblique light (thrown from all sides, as when the condenser and a 

 central stop are used) will vary ; but taking the case of extreme 



