Royal Society, f\ 



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 

 obliquity of the rays, the lens will only be visible by a luminous mar- 

 gin from reflexion, giving it a very beautiful annular appearance. 

 Hence it is more distinct by direct, or slightly oblique, than by very 

 oblique light. 



But in certain objects, the irregularities of structure are of such 

 extreme minuteness, or the difference of the refractive power of the 

 various portions of the structure is so slight, that the course of the 

 rays is but little altered by refraction on passing through them, and, 

 under ordinary illumination, all the rays will enter the object-glass ; 

 neither are the rays accumulated into little cones or parcels, of suffi- 

 cient intensity to map out the little light or dark spots in the field 

 of the microscope, according to the relation of their foci with that 

 of the object-glass. 



Let us take the instance of an object with minute depressions on 

 the surface, as the valve of a Gyrosigma. These are so minute, that 

 when the light reflected from the ordinary mirror is used, the rays 

 passing through the depressed and the undepressed portions, are not 

 sufficiently refracted to cause either set to be excluded from the 

 object-glass, consequently both sets will enter it. The slightly 

 oblique and converging rays passing through a portion of the 

 valve become separated into two sets, one passing through the 

 thinner depressed portions, the other through the thicker and unde- 



