176 Phenomena of the Podura Test. 



beaded ribs lie altogether above the exclamation marks or clubs. 

 " The varicose ribbings are first seen, and the exclamation marks do 

 not become distinct till after the scale is focussed through." The 

 ribbing or streak is described by Mr. Beck in his work on the micro- 

 scope, and declared by him also to be out of focus, a drawing of 

 which is given (Plate Yll., Fig. 4).* It was not then suspected 

 that the true structure of the scale was put out of court, as it were, 

 and that to see the " markings " distinctly we were obliged to focus 

 absolutely through the structures, justifying the Colonel's words, 

 " these exclamation marks are probably illusive or spurious ap- 

 pearances, notwithstanding the great distinctness they attain under 

 favourable conditions." In confirmation of this opinion I may be 

 permitted to state that these markings cannot be finely seen unless 

 the objective is spherically tinder-correded. 



V. If one examines with a pocket-lens the several sets of pho- 

 tographs, so beautifully done by the accomplished Colonel, and which 

 he has most handsomely placed at my disposal for reproduction, 

 we find numerous instances in the Podura Test-scale of beautifully- 

 formed spherical beads in close juxtaposition. It is difiicult to 

 imagine that difierent laws should govern the structure of the same 

 scale. If it is distinctly beaded in one part, one can scarcely with 

 propriety say it is merely varicose in another. 



POSTSCBIPT. 



Now that the sun is low in a wintry horizon and easily available 

 as a direct illuminator, I recommend, with great pleasure, to the 

 inquiring majority of our Fellows the following crucial experiment 

 on the Podura. Select a sunny aspect about 10.30 a.m. (with the 

 sun about S.E.) ; arrange a Podura scale, the best you have, with its 

 quill towards the S.E. ; then incline your microscope gradually till 

 the sun's uninterrupted rays impinge less and less obliquely upon the 

 under surface of the slide. You will find a position, according to the 

 angular aperture of your objective, when the scale becomes illumi- 

 nated on a darJc field. At that instant you will behold once for all, 

 if your glass be of first-rate quality, the whole scale studded uni- 

 versally with a molecular structure hitherto unsuspected ; but 

 revealing at a glance the poverty of old-fashioned definition. f 



* " The two appearances (Figs. 4 and 5) on one and the other side of the 

 focus when the adjustment of the object-glass is incorrect." TIio very accurate 

 drawing given unconsciously by Mr. Beck of the Podura riljbing is truly remark- 

 able. TJie exclamation markings are seen in a dilferent plane, and tlierefore out 

 of focus. Figs. 4 and 5, in Mr. IJeck's Plate VII., are well worth attention. 



t As a first essay tlie young microscopist may be recommended to experiment 

 at first upon the largely-beaded scales, as Podura vxicrotomi and Podura degeeria. 



