ANGULAR APERTURE. [ 



under which the object is examined in the 

 above experiments, involve simply the view- 

 ing of the object when illuminated entirely 

 or more completely by oblique light. For 

 an object-glass of large aperture admits more 

 oblique rays than one of less aperture, the 

 central rays being in nowise interfered with ; 

 inclining the mirror to one side, causes all the 

 rays which are reflected from it to become 

 oblique; and the use of central stops excludes 

 all the central rays, so that only the oblique 

 rays are admitted. Hence the visibility or 

 greater distinctness of the markings upon an 

 object depends upon its illumination by 

 oblique light. 



Experiment also shows us, that the degree 

 of obliquity of the light requisite, varies with 

 the delicacy or fineness of the markings, 

 being greater as these are more delicate ; so 

 that the most delicate markings require the 

 most oblique light which can possibly be 

 obtained, to render them evident, and the 

 angular aperture of the object-glass must 

 necessarily be proportionatelylarge, otherwise 

 none of these oblique rays could enter it. 



It has never yet been satisfactorily ex- 

 plained why transparent objects, which require 

 penetrating power in an object-glass, are best 

 shown, or can only be shown under the 

 microscope by oblique light. 



In entering upon this question, we may 

 take the opportunity of examining somewhat 

 minutely the reason why objects become 

 visible to us under various circumstances. 



The ordinary cause of objects becoming 

 visible to us under the microscope, is that a 

 certain number of the rays of light trans- 

 mitted through or incident upon them or 

 their parts, either become absorbed, refracted 

 or reflected. Thus the parts at which these 

 phenomena occur, may become either co- 

 loured or dark, whilst the parts which 

 transmit or reflect the light, become lumi- 

 nous. We shall leave the cases of absorption 

 and reflexion out of the question at present, 

 and consider only those of refraction. 



If the parts which refract the light are 

 large in proportion to the power of the object- 

 glass and of irregular form, they will refract 

 a certain number of rays so that these cannot 

 enter the object-glass, and they will hence 

 become dark, and will map out, as it were, 

 in the image formed of the object, the 

 structural peculiarities of the object. But if 

 the parts are minute, of a curved form and 

 approximatively symmetrical, they will act 

 upon the light transmitted through them in 

 the manner of lenses, and their luminous or 



] ANGULAR APERTURE. 



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 from the field of the 

 microscope ; also, from the concentration or 

 dispersion of portions of the light by these 

 parts, all the rays being admitted by the 

 object-glass, or entering the field. In speak- 

 ing of the parts being small or large, it must 

 be understood that the refractive powers of 

 the objects are assumed to be the same ; for 

 if the object be large and the substance of 

 which it is composed have a low refractive 

 power, the same effect may be produced as 

 if the object were small and of high refractive 

 power, the form being also the same. 



Another condition, rather physiological 

 than optical, is concerned in the question of 

 the distinctness with which an object is seen, 

 nay, even of its absolute visibility. It con- 

 sists in the relation which the luminousness 

 or darkness of an object bears to that of the 

 field or back-ground upon which it is appa- 

 rently situated; and all objects, even those 

 seen with the naked eye, may be regarded as 

 viewed upon a back-ground or field, compa- 

 rably to an object viewed in the field of the 

 microscope. The familiar instance of the 

 visibility of the stars by day from the bottom 

 of a coal-pit, whilst invisible from the surface 

 of the earth, may serve to illustrate this 

 point. The same phenomenon is constantly 

 met with in microscopic investigations; thus 

 it is well known that parts of structure which 

 are visible most clearly by the light of a lamp 

 in a dark room, cannot be distinguished 

 when the room is illuminated by ordinary 

 daylight; and luminous objects are best seen 

 on a black ground, and dark objects on a 

 light ground. 



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. 

 All that is required here is that the object- 

 glass shall be achromatic, and that the mar- 

 ginal rays shall not be decomposed, so that 

 any of the coloured rays should enter the 

 field; in which case, the margins of the 

 objects would appear coloured instead of 



