ANGULAR APERTURE. [ 



those rays admitted by that of less aperture, 

 and a certain number of other rays, these 

 being more oblique. 



Measurement of the angle of aperture. It 

 is of the utmost importance to know the 

 angle of aperture of the object-glasses used 

 in investigations ; because 'the appearances 

 presented by objects vary according to the 

 magnitude of this angle, and this variation 

 must always be taken into account in de- 

 termining the structure of an object from 

 its appearance. A particular piece of appa- 

 ratus is requisite for this purpose (fig. 18), 



Fig. 18. 



which may easily be constructed as follows. 

 A rectangular piece of board must be pro- 

 cured, the shortest sides of which are about 

 2 inches longer than the body of the micro- 

 scope, and the longer sides twice this length. 

 A small hole must then be made opposite 

 the middle of one of the long sides, at about 

 half an inch from its margin, and from this, 

 as a centre, a semicircle must be traced 

 upon the board, and the semicircular line 

 divided into 180 ; the portions outside the 

 semicircle being cut away. The wooden 

 plate of this form is shown in perspective in 

 the woodcut. A flat thin piece of wood () 

 rather broader than the body of the micro- 

 scope, a little longer than the radius of the 

 semicircle and pointed at one end, is then 

 placed upon the board in such a manner that 

 the pointed end corresponds with the gra- 

 duated margin, whilst the other end is 

 transfixed by a pin (b) which below is driven 

 into the board. Thus we have a rotating 

 arm or radius of the semicircle, which may 

 be compared to the hand of a watch or clock, 

 the pin forming the centre of rotation. To 

 the upper surface of this arm are glued two 

 thin pieces of wood, excavated in the 

 middle, so as to form supports for the body 

 of the microscope ; the excavations should 

 be triangular, the apex being directed down- 

 wards. 



] ANGULAR APERTURE. 



When used, the object-glass to be tested 

 is screwed to the end of the body next the 

 pin, and so adjusted that its focal point is 

 as nearly as possible perpendicularly over 

 the pin. A lamp is placed 2 or 3 yards 

 from the board and upon exactly the same 

 level as the axis of the body of the micro- 

 scope, 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 placed so 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 eyepiece 

 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 coinciilcs 

 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. 



This is the old method (Lister's), and that 

 by which object-glasses are still generally 

 measured; so that, for the purposes of 

 comparison, it will be found useful. 



But it is considered that it gives an 

 erroneous excess of aperture, from the ad- 

 mission of rays from lateral pencils, which 

 should be excluded. Wenham recommends 

 as a more accurate method, that half the 

 front lens of the object-glass be covered 

 with a piece of tin-fo'il. The degrees from 

 the half lens are then subtracted from those 

 of the entire lens, and twice the difference 

 represents the aperture. 



A very rough method of determining 

 whether an object-glass has a small or large 

 angle of aperture is this : direct the object- 

 glass, with the lower end towards the eye, 

 to the sky at a window ; then, keeping the 

 object-glass fixed, move the eye on one side 

 until light is no longer visible in the glass. 

 The extent to which the eye can be moved 

 round, before the light vanishes, indicates 

 the extent of aperture. 



Another method is described in Car- 

 penter's l Microscope,' 1868, p. 172, or Ro- 

 bin, 'Microscope,' 1877, p. 103; also Abbe's 

 " Apertometer," with figs., in the Jn. Micr. 

 Soc. 1878, i. p. 19; and H. Smith's "Univer- 

 sal Apertometer," in the Jn. Micr. Soc. ii. 

 1879, p. 775 (fig.). 



According to Abbe (Crisp), the aperture 

 of an optical instrument indicates a greater 

 or less capacity for receiving rays from 



