ORGANIC TEST-OBJECTS. 135 



come into view successively. The higher powers, which give an 

 amplification of 500 600 linear, exhibit the three series of lines 

 simultaneously with direct (axial) light, and resolve them with 

 oblique illumination into bright points, which are arranged in rows 

 in the three directions. With the best objectives it is possible to see 

 these bright points resolved as somewhat irregular hexagons, the 

 two longest sides of which are parallel to the median-line (Fig. 83). 

 The dark lines, which with lower power appear perfectly straight, 

 are therefore in reality zig-zag lines, and the tw r o oblique series 

 consist, as shown in the figure, of elements running alternately 

 lengthwise and obliquely, whilst the transverse lines are formed 

 only by the latter. Since the elements 

 running lengthwise are throughout 

 coarser than the oblique ones, the 

 greater clearness of the oblique series 

 of markings is accounted for. Their 

 apparent difference of level is due 

 merely to an optical effect, which is 

 explained by the unequal position of the 

 oblique strite in relation to the incident FlG ' 83 * 



light 1 , hence one and the same series of stria?, with a given 

 position of the midrib, appears to lie decidedly higher, but when 

 rotated about 90, just as decidedly lower. 



This excellent test-object, and the others we have described, can 

 be obtained from Bourgogne, of Paris, and Moeller, of Wedel 

 (Holstein), and also from most of the opticians. The specimens 

 are generally mounted in balsam, and are eo numerous that one 

 suitable for observation may be quickly found. The larger ones 

 exhibit, as might be expected, somewhat more distinct markings 

 than the smaller ones. The microscopist ought, however, to be 

 satisfied if the Microscope resolves the largest and finest speci- 

 mens in the manner above described. 



6. Plcurosiynia attenuatum. With strong longitudinal striae 

 parallel to the midrib, and fine transverse striae at right angles. 



according to the power of the objectives ; objectives with a focal length of 4 to 

 5 mm. should show the markings with favourable eye-piece amplification. 



1 For, since the coarser elements, situated lengthwise, belong to the two 

 oblique systems of markings, and therefore act similarly in every position 

 towards the incident light, the difference in the effect can arise only from the 

 obliquely-situated elements. 



