ANGULAR APERTURE. [ 



black, and thus the contrast requisite for 

 distinctness would be lost. 



The cause of the distinctness of an object 

 by refraction, all or nearly all the rays enter- 

 ing the field of the microscope, may be inves- 

 tigated 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 exerts the 

 action of 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 always 

 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 distinguish- 

 able by the above focal relation. Its appear- 

 ance 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 margin from 

 reflexion, giving it a very beautiful annular ap- 

 pearance. 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 sufficient 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 Gyro- 

 sigma. These are so mi- 

 nute, that when the light 

 reflected from the ordi- 

 nary mirror is used, the 

 rays passing through the 

 depressed and the unde- 

 pressed portions are not 

 sufficiently refracted to 

 cause either set to be ex- 

 cluded from the object- 

 glass, consequently both 



Fig. 19. 



] ANGULAR APERTURE. 



sets will enter it. This may be supposed 

 to be represented in fig. 19, where the 

 slightly oblique and converging rays passing 

 through a portion of the valve become sepa- 

 rated into two sets, one passing through the 

 thinner depressed portions, the other through 

 the thicker and undepressed portions ; both 

 sets enter the object-glass. But on trans- 



mitting oblique light through the object, as 

 represented in fig. 20, one set of the rays 

 will be refracted so as not to enter the object- 

 glass, whilst the other set will gain admis- 

 sion ; thus the two parts, which have differ- 

 ently refracted the rays, will become distinct. 

 If the markings were more delicate, or the 

 difference between the refractive power of 

 the two portions of the valve were less than 

 that represented in fig. 20, both sets would 

 enter the object-glass. But on rendering 

 the light still more oblique, one set would 

 be again excluded, from 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 to cause 

 the exclusion of one set of the rays will be 

 very great, and the other set will be too 

 oblique to enter the object-glass, unless it be 

 of correspondingly large aperture. 



This is the simple explanation of the 

 necessity for oblique light to render evident 

 the markings upon objects. 



No explanation of any peculiar agency by 

 which objects become more distinctly visible 

 when illuminated by oblique than direct 

 light is requisite, because it is not a fact that 

 such exists. The distinctness with which 

 objects or their parts are seen, always varies 

 according to the contrast they form with the 

 ground upon which they are viewed ; and in 



