TEST-OBJECTS. 



[ 637 ] 



TEST-OBJECTS. 



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

 on deal (PI. 1. fig. 4); the coarser scales of 

 Lepisma (PI. 1. fig. 6«); the pygidium of 

 the flea (PI. 1. fig. 13 «, b), the entire struc- 

 ture visible under the high eyepiece; a 

 dark scale of Podura (PI. 1. fig. 12 6). 



i-inch object-glass. Magnifying power 

 220 diameters; angular aperture 75 to 140°. 



Tests : hair of Dermestes ; the disks of deal; 

 the salivary corpuscles (PI. 1. fig. 5), the 

 moving molecules being clearly distinguish- 

 able ; the smaller scales of Lepisma (PI. 1. 

 fig. 6 a, b); the scales of Podura ; the fila- 

 ments of Didymohelix (PI. 1. fig. 10 «) ; 

 the pygidium of the flea, and the scales of 

 Pontia brassicm (PI. 2/. fig. 24). 



^-inch object-glass. Magnifying power 

 420 to 450 diameters; angular aperture 110 

 to 150°. 



Tests : the paler scales oi Podura; the pygi- 

 dium of the flea ; the scales of Pontia brassi- 

 ccb; the filaments oi Didymohelix, showing the 

 component fibres ; the salivary corpuscles. 



■Y^.jth or -^-gth-inch object-glass. Magni- 

 fying power 600 to 650 diameters ; angular 

 aperture 80 to 120°. 



Tests : the paler scales of Podura ; the fila- 

 ments of Didymohelix mounted in balsam ; 

 and the primitive fibrillae of muscular fibre 

 (PI. 17. fig. 36 6, rf). 



It will be observed that we have omitted 

 the tests for angular aperture, which many 

 of our microscopists look upon as the true 

 tests of the value of an object-glass. Our 

 reasons for this are given in the Introduc- 

 tion (p. xv). Those, however, who wish 

 for an interesting series of difficult objects 

 in this respect, will find one in the valves of 

 Gyrosigma, Grammatophora, Fragilaria, 

 Rhipidophora, Amphipleiira, some species of 

 Nitzschia, as N. tcenia, and Berkeley ia (see 

 these articles). We regard large angular 

 aperture in an object-glass as of little im- 

 portance ; because it is only of service for 

 showing the markings upon the valves of 

 the Diatomaceae, and the time is probably 

 near at hand when the presence and size of 

 these will be shown to possess neither 

 generic nor specific importance ; moreover, 

 object-glasses of large aperture and high 

 power approach so nearly to the object, that 

 they are inapplicable to important physiolo- 

 gical investigations. 



We shall now ofi'er 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 tolerable magni- 

 fying power. 



There is also some difficulty to an un- 

 practised eye in discriminating betw'een 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. The best plan in 

 regard to this point is to select an object, as 

 the scale of an insect or whatever it may be, 

 in which the test-structure is not distin- 

 guishable under the next highest power, and 

 then to examine the same object under the 

 power to be tested. 



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 obseiTer. They 

 may be obtained from Mr. Norman, Fountain 

 Place, City Road ; Mr. Topping, New Win- 

 chester Street, Pentonville; or of Messrs. 

 Smith and Beck, Coleman Street, City. 



A few^ notes upon the test-objects them- 

 selves may not be out of place here. 



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

 should be mounted in Canada balsam. Many 

 of those represented in PI. 22 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 un- 

 corrected chromatic aberration are easily 

 seen with a bad object-glass. 



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

 Obtained 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; PL 27. 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 obhque 



