TEST-OBJECTS, 



[ 761 ] 



TEST-OBJECTS. 



(fig. 13 a, h), the entire structure visible 

 under the high eyepiece ; a dark scale of 

 iWjim {PI. 1. fig. 12 6). 



\-inch oI)ject-ijlass. Magnifying power 

 220 diameters. 



Tests : liair of Dermestes ; the disks of 

 deal; the salivary corpuscles (PI. 1. tig. 5), 

 the moviuo; molecules being clearly distin- 

 guishable ; tlie smaller scales of Li'pisma 

 (PI. 1. fig. 6 b, c) ; the scales of Pudura ; 

 the filaments oi Didi/mo/ielix (fig. 10 a) ; the 

 pygidium of the Hea, and the scales of 

 Fonfia brassicce (PI. Si. fig. 24). 



Ifh-inch objed-glass. Magnifying power 

 420 to 450 diameters 



Tests : the paler scales of Podura ; the 

 pygidium of the flea ; the scales of Pontia 

 brussicfs ; the filaments of Didymohelix, 

 showing the component fibres ; the salivary 

 corpuscles. 



^\ih or ^\fh-inch object-glass. Magnify- 

 ing power GOO to (3o0 diameters. 



Tests : the paler scales of Podura ; the fila- 

 ments of Didj/ino/i^lix mounted in balsam ; 

 and the primitive tibrillae of muscular fibre 

 (PI. 22. fig. .36 6, rf). 



,';</* a?ul upioards. Magnifying power 

 1250 to 2000 diameters. 



Tests : the more delicate of the above 

 obiects, and the finest Nobert's lines. 



It wiU be observed that we have omitted 

 the tests for angular aperture, which are 

 often now regarded as the true tests of the 

 value of an object-glass. Those, however, 

 who wash for an interesting series of difficult 

 objects in this respect, may use that of 

 Dippel, viz. : the valves of Pleurosigma for- 

 mostim, attenuatum, anyulatum ; Grainmato- 

 2)1wra marina ; NitzscMa mjmoidea ; Navi- 

 cula rhomboides ; N. affinis, Amici's test- 

 object, that used at the Exhibition of 1802, 

 mounted in balsam, the transverse lines ; 

 Surirella gemma, the longitudinal lines; 

 Grammatophora subtilissima ; Frustulia 

 saxonica, and Bailey's Hyalodiscus subti- 

 liss{7nus. Nitzschiatcenia and Berkeleya fra- 

 gilis are also fine objects, MoUer's or 

 Rodig's Test-plates may also be used. These 

 consist of a series of the valves of Diatoma- 

 ceae, arranged according to their difficulty, 

 and moimted on a glass plate in balsam. 

 We shall now offer a few 

 General remarks on the application of 

 test-objects to the choice of an object-glass. 

 A gi-eat difficulty presents itself in this ques- 

 tion in the case of persons commencing the 

 use of the microscope ; for on viewing 

 almost any object, they will see so much 



that was invisible before, that they are natu- 

 rally led to regard an object-glass as good 

 which may simply possess tolerably magni- 

 fying power. 



There is also some difficulty to an un- 

 practised eye in discriminating between a 

 well-defined margin of an object, and one 

 which is ill-defined. This may be overcome 

 by purchasing one or two test-objects from 

 those who mount objects for sale, and first 

 viewing them under their microscopes; or 

 by examining some of the objects exhibited 

 at the evening, meetings of the learned 

 societies. 



The objects themselves are also variable, 

 some being much more delicate than others 

 even of the same kind. Moreover objects 

 viewed under immersion-glasses are more 

 brilliant than under the dry object-glasses. 

 The manner in which objects are mounted 

 is also of importance; for if they be immersed 

 in too much balsam or covered by too thick 

 a cover, no object-glass will show them well, 

 however good it may be. Hence the ne- 

 cessity of purchasing the test-objects, in the 

 case of an inexperienced observer. Any of 

 these may be obtained from Norman, Foun- 

 tain Place, City Road; Wheeler, 48 T(j1- 

 lington Road, HoUoway ; Baker, 244 High 

 Holborn ; of Smith and Beck, Ross, or 

 Powell ; or Bourgoyne, of Paris, 



A few notes upon the test-objects them- 

 selves may not be out of place here. 



Hairs of animah (PI. 1. figs. 1-3). These 

 should be mounted in Canada balsam. Many 

 of those represented in PI. 29 might be used 

 with equal advantage. 



Disks of deal (PI. 1. fig. 4), fonn a good 

 test-object on account of their freedom from 

 colour, whence the colours from uncor- 

 rected chromatic aberration are easily seen 

 with a bad object-glass. 



Salivary globules (PI. 1. fig. 5 a, b, c). Ob- 

 tained from the saliva. A good test-object 

 for those engaged in physiological investi- 

 gations ; the marginal granules and the 

 moving molecules should be very distinct, 



Scales of insects (PI. 1. figs. 6 «, b, c, 12 a, 

 b, c; PI. 34. fig. 24), These should be 

 mounted dry. The scales of Tinea and many 

 others have nothing to recommend them. 

 Nor do we advise the use of those scales 

 which exhibit the transverse stria? by oblique 

 light, as those oi Morpho (PI. 1. fig. 7), of 

 Hipparchia (fig. 9), Sec, as they are easy 

 tests even to inferior English object-glasses 

 of the present day. The long scales of 

 Pontia brassicce, however, are good. 



