TEST-OBJECTS. 



[ 636 ] 



TEST-OBJECTS. 



croscopic objects used to determine the value 

 of object-glasses. 



We must presume that the reader has 

 perused the remarks upon object-glasses in 

 the Introduction (p. xiv) ; also the article 

 ANGULAR APERTURE; otherwise the obser- 

 vations made here will be unintelligible. 



The main points in which object-glasses 

 differ from each other are four ; viz. 1. their 

 magnifying power ; 2. their defining power ; 

 3. their penetrating power ; and 4. their cor- 

 rective adaptations. 



1. The magnifying or separating power 

 scarcely requires notice ; it must be adapted 

 to the size of the objects likely to come 

 under examination. Usually, several object- 

 glasses are kept, of different powers ; at all 

 events, if scientific investigations are to be 

 pursued, a power of 400 diameters must be 

 accessible, and this without the use of the 

 highest eyepiece. The magnifying power 

 should be ascertained by MEASUREMENT, 

 and not by judging from the focal length. 



2. Good defining power is the most im- 

 portant character of an object-glass ; and if 

 good in respect to this, the dark boundary 

 lines of the test-objects will appear clear, 

 black, sharp, as if engraved, and quite free 

 from colour. If this is ascertained to be the 

 case, the higher eyepieces should be put on; 

 and it must be observed, that although the 

 sharpness of the outline is somewhat dimi- 

 nished, all the parts are clearly distinguish- 

 able as before. In this examination the 

 light should be as direct as possible. 



3. The power of displaying the minute 

 structural peculiarities of objects, or the pe- 

 netrating power, as it is called, depends upon 

 two distinct circumstances ; the goodness of 

 the defining power and the magnitude of 

 the angular aperture of the object-glass : 

 the degree of obliquity of the light is also of 

 great importance in connexion with the 

 latter. 



Thus, in examining the scale of a Podura 

 (PI. 1. fig. 12 a, 6, c), the magnifying power 

 being sufficiently high, if the defining power 

 be good, the wedge-shaped bodies will be 

 clearly and sharply displayed by direct light, 

 and whether the angular aperture be large 

 or small. Now, if we examine a valve of 

 Gyrosigma (PI. 1. figs. 17 & 18) by direct 

 light, the minute structure will be invisible, 

 however small or large the angular aperture 

 may be, or however perfect the defining 

 power ; but if the light be thrown obliquely, 

 and the aperture be sufficient, the striae will 

 at once become evident. Hence there are 



two distinct kinds of penetrating power, one 

 of which is the same as the defining power, 

 the other depending upon a different cause, 

 and hence the term penetration or penetra- 

 ting power should be laid aside, as tending to 

 cause confusion, the properties of object- 

 glasses being reducible simply to their defi- 

 ning power and their angular aperture. 



The defining power should be tested upon 

 the different objects mentioned below in con- 

 nexion with each object-glass, and the angular 

 aperture should be determined by measure- 

 ment (ANGULAR APERTURE), for judgment 

 founded upon the examination of the valves 

 of the Diatomaceae may be very fallacious to 

 an unpractised observer, on account of the 

 influence of the obliquity of the light, and 

 of the correcting adjustment. If, however, 

 an opinion is to be formed in this way, the 

 valves should be examined by oblique light 

 thrown from all sides, as with the central 

 stop in the condenser, so that the dots may 

 be viewed; for an object-glass may show 

 the lines very fairly, but the dots very 

 badly. 



4. The correcting adjustment is of im- 

 portance in examining very delicate objects 

 or structures with the high powers; it should 

 therefore always be present. 



We subjoin, in connexion with each object- 

 glass, the magnitudes of the angle of aper- 

 ture which they usually have in this country, 

 and which may be regarded as standards for 

 comparison; also those objects which will 

 be found most suitable for the purpose of 

 testing an object-glass. 



H or 2-inch object-glass. Magnifying 

 power 20 diameters ; angular aperture 12 to 

 20. 



Test-objects : the pygidium of the flea 

 (PL 1. fig. 13 a), in which the general out- 

 line and the hairs should be distinct ; the 

 hair of the mouse (PI. 1. fig. 3). Also, as 

 an opake object, a piece of an injected pre- 

 paration (PL 31. figs. 33-35). 



\-inch or %rds-object-glass. Magnifying 

 power 60 diameters ; angular aperture 22 

 to 27. 



Tests: hair of Dermestes (PL 1. fig. 1) ; 

 of the bat (PL 1. fig. 2); of the mouse (PL 

 1. fig. 3); the pygidium of the flea, the 

 outline of the areolae being distinguishable 

 under the high eyepiece (120 to 200 dia- 

 meters), but not the rays. Also an injection, 

 as a piece of lung. 



\-inch or -^ths-inch object-glass. Mag- 

 nifying power 100 to 120 diameters ; angu- 

 lar aperture 55. 



