TEST-OBJECTS. 



[ 761 ] 



TEST-OBJECTS. 



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

 under the high eyepiece ; a dark scale of 

 Podura (PI. 1. fig. 12 A). 



\-inch object-glass. Magnifying power 

 220 diameters. 



Tests : hair of Dermestes ; the disks of 

 d"al ; the salivary corpuscles (PI. 1. fig. 5), 

 the moving molecules being clearly distin- 

 guishable ; the smaller scales of "Lepisma 

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

 the filaments of Didymohelix (fig. 10 ) ; the 

 pvgidium of the flea, and the scales of 

 PtotfM brassica (PI. 34. fig. 24). 



fy.h-inch object-glass. Magnifying power 

 420 to 400 diameters 



Tests: the paler scales of Podura; the 

 pvgidium of the flea ; the scales of Pontia 

 brd*xic<e ; the filaments of Didymohelix, 

 showing the component fibres ; the salivary 

 corpuscles. 



i\th or jgth-inch object-glass. Magnify- 

 ing power (300 to 650 diameters. 



Tests : the paler scales of Podura ; the fila- 

 ments of DidymoJielix mounted in balsam ; 

 and the primitive tibrillae of muscular fibre 

 (PL 22. fig. 366, d). 



,'jtfA and upwards. Magnifying power 

 1260 to 2000 diameters. 



Tests : the more delicate of the above 

 objects, and the finest Nobert's lines. 



It will 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 wish for an interesting series of difficult 

 objects in this respect, may use that of 

 Dippel, viz. : the valves of Pleurosigma for- 

 mosum, attenuatum, angulatum ; Grammato- 

 phora marina ; Nitzschia sigmoidea ; Navi- 

 cufa rhomboides ; N. ajfinis, Amici's test- 

 object, that used at the Exhibition of 1862, 

 mounted in balsam, the transverse lines; 

 tSurireUa gemma, the longitudinal lines; 

 Grammatophora subtilissima ; Frmtulia 

 saxonica, and Bailey's Hyalodiscus subti- 

 lissimus. Nitzschia tania and Berkeley a fra- 

 f/i/is are also fine object 1 ?. Holler's or 

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

 consist of a series of the valves of Diatoma- 

 cene, arranged according to their difficulty, 

 and mounted 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 great 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-detined 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 Tol- 

 lington Road, Hollo way; 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 animals (PI. 1. tigs. 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), form 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, i, 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 a, 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 striae by oblique 

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

 Hipparchia (fig. 9), &c., as they are easy 

 tests even to inferior English object-glasses 

 of the present day. The long scales of 

 Pontia brassicce, however, are good. 



