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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- 

 pressed portions: still both sets enter the object-glass. But on trans- 

 mitting oblique light through the object, one set of the rays will be 

 refracted so as not to enter the object-glass, whilst the other set will 

 gain admission ; thus the two parts, which have differently refracted 

 the rays, will become distinct. If the markings were more delicate, 

 or if the difference between the refractive power of the two portions 

 of the valve were less, both sets would enter the object-glass. But 

 on rendering the light still more oblique, one set would be again 

 excluded by being refracted out of the field. Hence it is evident 

 why the angular aperture of the object-glass must be larger as 

 the markings are finer, or the difference between the refractive 

 power of the two portions of tissue is less ; because the obliquity 

 of the light requisite will be very great to cause the exclusion 

 of one set of the rays, and the other set will be too oblique to 

 enter the object-glass unless it be of correspondingly large aperture. 

 This is the explanation of the advantage of oblique light. It has 



