136 



TESTING THE MICROSCOPE. 



Longitudinal strife with direct light appear distinct with a power 

 of 150 200 linear ; for instance, with Beneche's No. 7 objective 

 and No. 1 eye-piece, transverse strife are shown in places distinctly 

 with direct (axial) light with an amplification of about 300, and with 

 oblique illumination very sharp throughout, with Beneche's No. 9 

 and Hartnack's No. 7. With higher power objectives the bright 

 lines are resolved, as occurs always with similar markings, very 

 distinctly into small squares or, more accurately, rectangles, whose 

 angles are more or less truncated, so that the dark division-lines 

 appear thickened into knots. 



7. Pleurosigma balticum. With longitudinal strife, and rather 

 fine transverse strife ; the former visible with a power of 200 linear 

 in oblique light, the latter scarcely distinct with 300. Beneche's 



No. 7 shows the longitudinal lines 

 separated only with the deeper 

 eye-pieces, and the transverse 

 lines on the margin ; with Hart- 

 nack's No. 9 and No. 7 both series 

 are resolved distinctly. 



8. Grammatoplwra marina. 

 Marginal parts with transverse 

 stria 1 , and two less distinct oblique 

 series of strife as in Pleurosigma 

 angulatum. The three series of 

 stria3 can be seen with Beneche's 

 No. 9, and even the transverse 

 strife with the usual eye-pieces 

 Nos. 1 and' 2, but to see the 

 oblique ones clearly deeper eye-pieces are required. Hartnack's 

 No. 7 gives about the same results. We may therefore insist that 

 a Microscope should at least exhibit the three series of strife with 

 an amplification of 340 400 linear. 



9. Swrirdla Gemma (Fig. 84). Between the strong cross-bars 

 which appear at irregular distances on both sides of the midrib, 

 and parallel with them, are fine transverse strife, which are them- 

 selves intersected at right angles by exceedingly fine longitudinal 

 strife (Fig. 85). These latter are distinctly seen only with the 

 higher powers, such as Hartnack's Nos. 9 and 10; the former, 

 however, require merely a magnification of 400 500. With the 

 best objectives that are now manufactured the fine longitudinal 



FIG. 84. 



FIG. 86. 



