ANGULAR APERTURE. 



[ 39 ] 



ANGULAR APERTURE. 



the illumination of objects by oblique light 

 under the microscope, the manner in which 

 the rays are refracted or reflected by their 

 various parts, will always afford a satisfactory 

 explanation of the appearances they present 

 in accordance with the laws of refraction or 

 reflexion. 



The most difficult point to explain, has been 

 how it is that an object-glass of large angular 

 apertui'e will render markings evident which 

 were not visible under an object-glass of 

 smaller aperture ; because it would naturally 

 be imagined that the larger aperture would 

 admit both sets of rays (fig. 20), one of which 

 was excluded by the object-glass of smaller 

 aperture. The difficulty vanishes when it is 

 recollected that the additional rays admitted 

 by the object-glass of larger aperture are 

 more oblique; hence one set of these rays 

 ^Aill be refracted from the field of the micro- 

 scope, whilst the other set will enter the 

 object-glass and will illuminate the more 

 highly refi'active parts of the object; thus 

 the two kinds of differently refractive struc- 

 tm'e become distinctly separated, one appear- 

 ing dark, the other luminous ; and thus we 

 illuminate one part of the object, whilst the 

 ilhimination of the other is not increased. 

 Or, to simplify this very important point, the 

 object may be regarded as illuminated by 

 two sets of rays, one corresponding to those 

 admitted by the object-glass of small aper- 

 ture ; the other set con-esponding to these 

 plus those admitted by the excess of angular 

 aperture of the second object-glass over the 

 first; the first set of rays not being suffi- 

 ciently obUque to allow of a portion of them 

 being refracted beyond the angular aperture 

 of the first object-glass, whilst the second set 

 are so. Hence under the object-glass of 

 larger aperture, the distinctness of the mark- 

 ings is impaired by the admission of the first 

 set of rays. Now this always occurs when 

 objects are examined under an object-glass 

 of large aperture; although the more oblique 

 rays render the markings visible, by their 

 illuminating one part of the object and not 

 the other, the less oblique rays render them 

 indistinct by illuminating both parts, unless 

 the central stop be used, which totally inter- 

 cepts all but the very oblique rays, and allows 

 the markings to be seen in perfection, as 

 illuminated by the more oblique rays alone. 



Refraction of the rays of light has alone 

 been considered thus'^ fai* ; the action of 

 obhque light and angular aperture in relation 

 to colour and reflexion has been neglected. 

 The question of colour is easily answered. 



Neither oblique light nor large angular aper- 

 ture possesses any power of rendering co- 

 lom^ed transparent objects more distinct; and 

 markings, when arising from the presence of 

 pigment, are perfectly visible under an object- 

 glass of small apertm^e, and the ordinary 

 light of the mirror. 



It has been assumed that the oblique 

 light requisite for the display of the markings 

 upon objects is separated into two sets of 

 rays by refraction; but it might be ques- 

 tioned whether these are not separated by 

 reflexion. There can be no doubt that the 

 latter is not generally the case ; perhaps the 

 most important reason which may be assigned 

 for this is, the considerable comparative 

 breadth of the luminous portions, of the 

 valve of the Gyrosigma for instance. On 

 transmitting unilateral light obliquely 

 through the valve of an Isthmia, in which the 

 depressions are very large, in such manner 

 that part of it is reflected by portions of 

 them, it is easily seen how small the amount 

 of reflected light is ; and this because the 

 surface of the depressions is curved, and thus 

 the portions inclined at the requisite angle 

 for reflexion are also very small. As the 

 amount of light reflected is so small in this 

 case, it would be inappreciable in that of the 

 Gyrosigma, in which the depressions are so 

 exceedingly minute. In fact, attention to 

 this point afi^ords a ready means of distin- 

 guishing whether an object is illuminated by 

 reflexion or refraction. The illumination of 

 the sides of globules of oil by reflexion, has 

 been already noticed at p. 38. The same 

 phaenomenon may be seen in the instance of 

 minute globules of metallic mercury. It 

 might be questioned whether the luminous 

 ring sm'rounding the globules in both cases 

 did not arise from diffi'action. It is, however, 

 far too defined for this ; and it cannot arise 

 from refraction in the latter case, because 

 the globules of mercury are opake. 



It has been shown, that the efficacy of 

 large angular aperture depends upon the 

 illumination of the objects by oblique light ; 

 and that the action of oblique light depends 

 upon one set of rays being refracted from 

 the field. Let us now attempt to trace the 

 relation of the penetrating power of an 

 object-glass to its defining power. It has 

 been stated, that penetrating power depends 

 upon angular aperture ; and as angular 

 aperture owes its efficacy to oblique light, 

 the relation of oblique light to penetration is 

 evident. Is there any essential difference 

 between penetrating and defining power? 



