1 72 Phenomena of the Podura Test. 



distance of ten inches, i. e. with an angular aperture of about one 

 degree* 



The club mark or note of admiration is therefore entirely different 

 in its nature from a diatomic line-mark primarily made out by a third- 

 rate objective. For these lines cannot joossibly be defined with so 

 small an aperture even as 15^: perhaps 30^ is the limit. In pro- 

 portion as the angle is enlarged, caeteris ixiribus, diatom markings 

 come out more and more distinctly. This broad fact points to the 

 conclusion that these Podura marks are shadoics ; they disappear 

 with oblique light. Parallel rays perpendicular to the scale or 

 coincident with the instrumental axis display them most clearly. A 

 great variety of such shadows or obstructions of light may be shown. 



(a) If a number of the Formosum diatoms be selected which 

 lie across each other irregularly, a great number of black shadow- 

 patterns are developed, easily seen with a low aperture and power, 

 many of them very closely imitating the Podm'a marks • or clubs. 

 Of course where light cannot get through on account of intricate 

 internal refractions and reflexions in a mass of brilliantly hght- 

 transmitting spherules, we get darkness or black markings. 



(6) If we dihgently search among shdes rich in overlying ribbed 

 scales, with a low power, we shall be rewarded with beautiful dark- 

 ness-patterns in many forms ; and the more nearly the ribbings of 

 the superimposed scales are parallel to each other the longer appear 

 the markings of the wavy group. 



(c) AYheuever minute cyhnders of a light-refracting character lie 

 athwart each other, black shadows are shown" near the points of 

 each intersection : with splendid definition these black shadows can 

 be discerned even in the plume-hairs of the gnat at their points of 

 intersection. Spun glass shows them in the highest perfection. 



I regard, therefore, the black marks on the Podura seen with a 

 low power and small angular aperture as simply jet-black shadows 

 from the stoppage of light-transmission. 



Some allusion must now be made to the phenomena approxi- 

 mately exhibited by minute transparent cylinders intersecting each 

 other. 



Sir David Brewster improvised fairly good lenses by placing 

 two small cylindrical bottles filled with water at right angles to each 

 other ; their intersection formed a lens which produces a fair image 

 of a distant object. In October, 1870, Mr. Moginie procured for 

 me a number of fine glass rods ; when these are placed in contact in 

 one plane, and another similar set is also placed behind them at 

 ri^ht angles, brilHant images of a distant flame are beautifully dis- 



* In other words the base of a triangle, with the pupil of the eye for base 

 and a point 10 inches distance for the apex, would subtend nearly one degree. 



