ANGULAR APERTURE. 



[ 35 ] 



ANGULAR APERTURE. 



3. Anff. tritici (PI. 16, fig. 6). 20 times as 

 long as broad in the adult state; length 1-42 

 to 1-4". 



Found in blighted wheat. 



4. Ang.glutiiiis{V\. 16. fig. 7). About 20 

 times as long as broad, terminating posteri- 

 orly in a fine elongated point; length 1-15". 



Found in sour paste. 



Other AnguiUulcB are found in the same 

 situations as A.fluviatills; but whether they 

 are distinct species, or merel}' varieties, has 

 not been determined. 



It is almost impossible to dissect these 

 minute beings in the ordinary manner ; the 

 best method of proceeding is to wound the 

 body, and gently press out the contents 

 under water. 



BiBL. Duges, Ann. d. Sc. Nat. 1826, ix. ; 

 Bauer, Phil. Trans. 1823 ; Ehrenberg, Sym- 

 bol(B PhysiccB {Phyt. Polypi) ; Dujardin, 

 Hist. d. Helminthes. 



ANGULAR APERTURE.— The angular 

 aperture of an object-glass is the angle 

 measured by the arc of a circle, the centre of 

 which is fomied by the focal point of the 

 object-glass, the radii being formed by the 

 most extreme lateral rays which the object- 

 glass admits. 



Thus let fig. 16 represent a perpendicular 

 section of the lowest combination of an ob- 

 ject-glass of small aperture ; 



Fig. 



16. 



Fig. 17. 



€ d 



a is the angle of aperture, and /, e the most 

 oblique rays which the object-glass will 

 admit ; the angle is measured by the dotted 

 arc h. In the object-glass of larger aperture, 

 fig. 17, the arc h which measures the angle 

 is much larger, and the radii representing 

 the extreme lateral rays are much more 

 oblique. Hence it is evident that the object- 

 glass of larger aperture admits all 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 frequently vary accord- 

 ing to the magnitude of this angle, and this 

 variation must always be taken into account 

 in determining the structure of an object 

 from its appeai'ance. A particular piece of 



Fig. 18. 



^5 



apparatus is requisite for this purpose 

 (fig. 18), which may easily be constructed as 

 follows. A rectangular piece of board must 

 be procured, the shortest sides of which are 

 about 2 inches longer than the body of the 

 microscope, 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 

 w^ood (a) rather broader than the body of the 

 microscope, a little longer than the radius of 

 the semicircle and pointed at one end, is 

 then placed upon the board in such manner 

 that the pointed end corresponds with the 

 graduated margin, whilst the other end is 

 transfixed by a pin (6) 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 tri- 

 angular, the apex being directed downwards. 

 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 



D 2 



