ANGULAR APERTURE. [ 36 ] ANGULAR APERTURE. 



board and upon exactly the same level as 

 the axis of the body of the microscope, the 

 straight side of the board being next the 

 lamp, and when the arm has been so adjusted 

 that the pointed end is opposite 90, the 

 lamp is so placed that the flame is seen 

 through the body of the microscope. The 

 eyepiece is next put into the other end. 

 The arm supporting the body of the micro- 

 scope is then moved on one side, the body 

 looked through in the usual manner, until 

 the field is seen to be divided into two parts, 

 a dark and a luminous half; the degree 

 which the pointed end of the arm coincides 

 with is then noted, and the arm is moved in 

 the other direction until the division of the 

 field is again seen ; the number of degrees 

 included in the arc, thus traversed, measures 

 the angle of aperture. 



It has been objected, that this method does 

 not afford an exact estimate of the angle of 

 aperture. But it is questionable whether 

 the objection made to it is real ; for it is a 

 fact that an object-glass, which, according 

 to the above method, is of larger aperture 

 than another, will display markings which 

 the one of less aperture will not. 



As an object-glass of large aperture admits 

 a greater number of oblique rays than one of 

 less aperture, the central rays being in 

 nowise interfered with, so the total number 

 of rays admitted is greater, and objects will 

 thereby be more brilliantly illuminated. This 

 is one of the advantages gained by the use of 

 an object-glass of large aperture; and the 

 explanation applies especially to its use in 

 the examination of opake objects, in regard 

 to which it can be readily understood that a 

 greater number of the rays reflected from all 

 parts of an object being admitted, will render 

 it more luminous and distinct. In this case 

 the same effect would be produced by con- 

 densing an additional amount of light upon 

 the object. 



But strictly speaking, large angular aper- 

 ture in an object-glass used in the examina- 

 tion of opake objects is disadvantageous; for 

 although objects thus viewed appear very 

 luminous, brilliant and beautiful, yet a 

 number of the rays which cannot enter an 

 object-glass of small aperture from their 

 obliquity, and which thus map out as it were 

 the form and structural appearances of the 

 object, are admitted by an object-glass of 

 large aperture, and thus the contrast by which 

 the various parts are rendered visible will be 

 destroyed. This applies especially to un- 

 coloured objects; for those which are 



coloured are best seen under a glass of larger 

 aperture, the difference between the tints of 

 colour reflected being sufficient to render 

 each part distinct. 



There is, however, another far more im- 

 portant use of large angular aperture in an 

 object-glass. It was first found by Dr. Go- 

 ring that longitudinal and transverse lines 

 upon the scales of Lepidopterous and other 

 insects could be seen under certain object- 

 glasses, but not under others ; and that the 

 power of displaying these, or the penetrating 

 power of the object-glass, as it is called, 

 depended upon the magnitude of the an- 

 gular aperture. The same has since been 

 found the case with the markings upon the 

 valves of the Diatomaceae. 



1. If the prepared valve of a Gyrosigma 

 be examined under an object-glass of 1-4 or 

 1-8 of an inch focus, and an angular aperture 

 of 60 or 70, as illuminated by the ordinary 

 light of the mirror, nothing more is seen 

 than the more or less coloured valve with a 

 distinct outline, the central line and the 

 nodules ; and no change is produced in the 

 appearances, however intensely the object 

 may be illuminated. But if an object-glass 

 of larger angular aperture be used, a number 

 of fine dark parallel lines are seen traversing 

 the valve. Hence the object-glass of larger 

 aperture possesses a particular power of ren- 

 dering indications of structure evident, w r hich 

 is not possessed by the one of less aperture. 



2. If, in the same experiment, the mirror 

 be brought towards one side of the stage, 

 and the light be then thrown upon the object, 

 the lines will become more distinct if pre- 

 viously visible, and frequently visible when 

 not so before. 



3. Placing a central stop in the object- 

 glass will also, to a certain extent, produce 

 the same effect as using the object-glass of 

 larger aperture. 



4. Placing a stop in the condensing lenses 

 of the achromatic condenser will increase the 

 distinctness with which the markings are 

 seen, if already visible, and will frequently 

 render them visible when not so before. 



These experiments show, that using an 

 object-glass of large aperture in the exami- 

 nation of an object, bringing the mirror to 

 one side, and placing a central stop in the 

 object-glass and the condenser, or in both, 

 produce the same effect, viz. that of render- 

 ing the markings upon an object visible when 

 not so previously, or of rendering them more 

 distinct if previously visible. And it is 

 evident that the alterations of the conditions 



