Phenomena of the Podura Test. 175 



with bright points distant from each other about the length of eight 

 or ten beads, according to the nature of the intersections. 



With my Powell and Lealand's 1-1 6th immersion I develop 

 alternate obliteration, and two short black lines intensely black, and 

 between them, beading. In fortunate cases both our old friends, 

 the clubs or shillelaghs, as well as two black lines and two different 

 sets of beads besides, are visible. 



1 have never been able to see these double short black lines in 

 any scales but PoduraB. 



Svhsidiary Forms ^presented hj the Podura Test. 



I. The lowest and worst form of development is the appearance 

 of a few isolated beads at some of the intersections of the molecular 

 ribbing. This can be readily produced by entirely deranging the 

 corrections of a superb objective. 



II. Another imperfect form of definition is a bulbous swelling, 

 like a knot in a vein. A varicose swelling in surgery is an irregular 

 bulging of the blood-vessel at irregular intervals. A few large and 

 smaller bullets forced into an india-rubber tube give the idea of 

 varicose swellings. But notwithstanding the frequent manner in 

 which my distinguished friend, Colonel Woodward, has applied the 

 term varicose as representing the appearance of the Podura ribbing, 

 this description seems to me hardly to apply to an apparent ribbi- 

 ness wholly composed of distinct beads closely packed together and 

 separable into individual molecules. In the varicose idea there is 

 continuity, in the "beaded form there is juxtaposition, like spheres 

 packed closely in an elastic and transparent tube, or like a necklace. 



III. A rippling undulation of headed ribs, as though a series 

 of necklaces were placed parallel upon the small ripples often seen 

 after the tide is out upon a sandy beach. This form appears by a 

 change in the cover correction. 



In observing Podura with the water lens and a cracked cover, 

 the fluid is seen to insinuate itself in straight lines, precisely following 

 the course of the beaded ribs ; bead after bead disappears and re- 

 appears under a dry lens, as the fluid gradually dries up. In some 

 cases I have observed pressure of the object-glass upon the slide 

 (delicately administered) to cause the escape of an oily fluid which 

 returns to the scale backwards and forwards as the pressure is 

 changed by the fine focussing adjustment, and the course of this oil 

 most clearly demonstrates the existence of level and not undulating 

 beaded ribbing. 



IV. The appearance of continuous ribs is the primary form of 

 development. Colonel Woodward, with that acumen and perspicuity 

 which so highly distinguish his communications, informs us that 

 he has observed this valuable fact (long known to myself) that the 



