58 



HARDWICKE'S SCIENCE-GOSSIP. 



[Mauch 1, 1SG7. 



distinct, and have a great tendency to follow each 

 other in longitudinal rows. In order to give a 

 rough idea of their appearance comparatively with 

 others we attach a figure, exhibiting them as seen 

 under an &th object-glass. In Mr. Beck's beautiful 

 paper in the Microscopical Journal, on the subject of 



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well with a good half-inch objective, assisted by the 

 condenser and a deep eyepiece ; but, although they 

 are said to be visible with a one-inch, it is only, to 

 quote from Dickens, when you " make believe very 

 much " that the mottlings then seen are comfortably 

 resolved. 





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Fig. 48. TestPodura Scale; upper portion, £ objective, 

 A eyepiece ; lower portion, & objective, C eyepiece. 



the Scale of Lepidocyrtus, he says : " The best scales 

 are obtained from the insects found in comparatively 

 dry places." So far, and in some other particulars, 

 I find our experiences agree. His speciality (which we 

 figure as the "test scale") is either a variety of the 

 black Podura or a distinct species ; most probably 

 the latter. The outlines of Podura scales are very 

 different in different species ; and although there 

 are many erratic shapes, those drawn will, we 

 think, be found to be the forms which predominate. 

 The drawings of the scales have all been made 

 with the aid of Ross or Beck's object-glasses and 

 Powell and Lealand's achromatic condenser (170°). 

 The markings can be seen without an achromatic 

 condenser; but the view is infinitely more satis- 

 factory when this apparatus is used. I have also 

 employed Smith and Beck's, which is admirably 

 adapted for exhibiting them. A moderate aperture 

 only is requisite, both in the object-glass and the 

 illuminating apparatus, for their perfect display ; but 

 the workmanship of the objective must be of the 

 best description. Any error in the correction of 

 the lenses, whether in the manufacture or in their 

 adjustment for penetrating the thin covering glass, 

 is immediately betrayed by the peculiar appearance 

 which the markings present. Mr. R. Beck has 

 dealt with this subject so thoroughly in the paper I 

 have alluded to, that I hardly feel competent to 

 speak on it* I have seen the markings moderately 



* I refer more particularly to the accepted standard test 

 scale. ' 



Fig. 50. Same 

 Scale, direct 

 illumination ; 

 T V objective, 

 C eyepiece. 



Fig. 49. Scale of Speckled Podura, oblique illumination. 



An object-glass, which will show the Podura scale 

 perfectly, may be predicted to be capable of being 

 employed satisfactorily on the easier diatoms, such 

 as P. angulata and P. hippocampus; but it does 

 not follow that another glass, which will show the 

 striae on the N. rhomboides, Surirella, P. fasciola, 

 N. cuspidata, &c, will perform equally well on the 

 Podura scale. It is possible, however, for a good 

 high-power glass to be equal to both these require- 

 ments. Amateur mounters should cover the scales 

 they put up with the thinnest possible glass, so that, 

 when opportunity offers for an examination of the 

 slide under a remarkably good -^th or T Vth, they 

 may not find themselves doomed to disappointment, 

 owiug to the inability of the objective to penetrate 

 the thick cover. 



In conclusion I beg to offer a few remarks on 

 the scale of the Speckled Podura. As I have 

 stated above, it possesses transverse striae, and 

 these are rendered most distinct when the central 

 rays of the achromatic condenser are stopped out. 

 I believe that the structure of Podura scales in 

 general may be best studied in this one. From very 

 careful examinations I have no doubt that the 



