TEST-OBJECTS. 



[ 760 ] 



TEST-OBJECTS. 



observations made here will be unintel- 

 ligible. 



The main points in which object-glasses 

 differ from each other are four : viz. their 

 magnifying power; their defining power; 

 their penetrating power ; and their correc- 

 tive adaptations. 



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 j udging from the focal length. 



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 diminished, all the parts are 

 clearly distinguishable as before. In this 

 examination the light should be as direct 

 as possible. 



The power of displaying the minute or 

 internal structural peculiarities of objects, 

 or the penetrating power, as it is called, de- 

 pends 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 con- 

 nexion with the latter. 



Thus, in examining the scale of a Podura 

 (PL 1. fig. 12 a, b, 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. But if we examine a valve of 

 Pkurosigma (PI. 1. tigs. 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. Thus 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 j hence the term penetration or pene- 



trating power should be laid aside, as 

 tending to cause confusion, the properties 

 of object-glasses being reducible simply 

 to their defining power and their angular 

 aperture. 



In recent times, a new meaning has been 

 assigned to the term penetrating power, 

 viz. that of rendering a certain thickness of 

 an object visible at one time. This de- 

 pends upon the suiallness of the angle of 

 aperture of the object-glass. It seeuis a 

 useless innovation. 



The defining power should be tested upon 

 the different objects mentioned below in 

 connexion with each object-glass, and the 

 angular aperture should be determined by 

 measurement (ANGULAR APERTURE) ; for 

 judgment founded upon the examination of 

 the valves of the Diatomacese 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 ba formed in this 

 way, the valves should be examined by ob- 

 lique 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. 



The correcting adjustment is of import- 

 ance in examining very delicate objects or 

 structures with the high powers ; it should 

 therefore always be present. 



We sub join "a list of a few of the objects 

 which will be found most suitable for the 

 purpose of testing an object-glass. 



1 or 2-inch object-glass. Magnifying 

 power 20 diameters. 



Test-objects: the pygidium of the flea 

 (PI. 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 opaque object, a piece of an injected 

 preparation (PL 39. figs. 33-35). 



\-inch or %rds object-glass. Magnifying 

 power 60 diameters. 



Tests: hair of Dcrmestes (PI. 1. fig 1); 

 of the bat (fig. 2) ; of the mouse (fig. 3) ; 

 the pygidium of the flea, the outline of the 

 areolse being distinguishable under the high 

 eyepiece (120 to 200 diameters), but not 

 the rays. Also an injection, as a piece of 

 lung. 



\-inch or fifths-inch object-glass. Magni- 

 fying power 100 to 120 diameters. 



Tests : hairs (PL 1. figs. 1, 2, 3) ; the disks 

 of deal (fig. 4) j the coarser scales of Le- 

 pisma (fig. 6 a) ; the pygidium of the flea 



