ANGULAR APERTURE. [ 46 ] ANGULAR APERTURE. 



obtained, to render them evident, and the 

 angular aperture of the object-glass must ne- 

 cessarily be proportionately large, otherwise 

 none of these oblique rays could enter it. 



In attempting to explain these pheno- 

 mena, we may take the opportunity of exa- 

 mining 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, re- 

 fracted or reflected. Hence the parts at 

 which refraction or absorption occurs may 

 become either coloured or dark, whilst those 

 which transmit or reflect the light become 

 luminous. We shall leave the cases of ab- 

 sorption and reflection out of the question 

 at present, and consider only those of re- 

 fraction. 



If the parts which refract the light are 

 large in proportion to the power of the ob- 

 ject-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 sha- 

 dow 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 approximative^ symme- 

 trical, their lenticular foci will produce a 

 luminous or dark appearance, according to 

 the relation of the foci to that of the object- 

 glass. Thus, the parts of an object may ap- 

 pear dark and denned, 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. 



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 invisibility. It 

 consists in the relation which the luminous- 

 ness or darkness of an object bears to that 

 of the field or background upon which it is 

 apparently situated ; and all objects, even 

 those seen with the naked eye, may be re- 

 garded as viewed upon a background or 

 field, comparably to an object viewed in the 

 field of the microscope. The familiar in- 

 stance of the visibility of the stars by day 

 from the bottom of a coal-pit, whilst invi- 

 sible 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 day- 

 light ; 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 im- 

 pair 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 ob- 

 jects would appear coloured instead of black, 

 and thus the contrast requisite for distinct- 

 ness would be lost. 



But in certain objects, the irregularities 

 of structure are of such extreme minute- 

 ness, or the difference of the refractive 

 power of the various portions of the struc- 

 ture is so slight, that the course of the rays 

 is but little altered by passing through 

 them; and under ordinary illumination, 

 all the rays will enter the object-glass; 

 neither are the lenticular foci sufficient to 

 map out the little light or dark spots in the 

 field of the microscope according to their 

 relation to that of the object-glass. 



Let us take the instance of an object with 

 minute markings on the 

 surface, as the valve of a 

 Pleurosigma. These are 

 so minute, that when the 

 light reflected from the 

 ordinary mirror is used, 

 the rays passing through 

 the depressed and the un- 

 depressed portions are not 

 sufficiently refracted to 

 cause either set to be ex- 

 cluded from the object- 

 glass, consequently both 

 sets will enter it (ng. 19). 

 But on transmitting ob- 

 lique light through the 

 object, as represented in fig. 20, one set of 



