Phenomena of the Podura Test. 173 



played, and as the angle at wbich the glass rods intersect is changed, 

 distorted images are formed of great variety* 



Now when a scale is composed of ribbing approximately cylin- 

 drical, and when there is a double set, the one crossing the other, as 

 in the Lepisma, an approximate definition shows a bead-like ap- 

 pearance at the points of intersection, as noticed long ago by Mr. 

 Beck : these ciuasi beads are limited to the points of intersection, and 

 are very S23arse and few. A more profound and recherche definition 

 dissipates these false beads and displays a surprising wealth of 

 beading composing the entirety of each Lepisma ribbing in both 

 sets (described by the writer in 1869). 



Molecular Definition. — Whenever molecules, or spherules, or 

 beads are arranged in straight linear rows and in close contact, 

 whether in diatoms or scales (before the glasses receive those ex- 

 quisite corrections necessary to disintegrate the cylindrical appear- 

 ance into its comjDonent beading), their primary appearance under 

 an approximate definition is always cylindricular, that is formed 

 of cylindricles. 



I may remark that some of the intersections on Lepisma near 

 the edge closely simulate Podura markings ; and the elongated beads 

 described by Mr. E. Beck in this Journal, resolve themselves into 

 straight clusters of minute beading ; similar to those of the resolved 

 Podura clubs or markings. 



The molecular definition of these bodies is clearly dependent 

 upon the definition of each molecule. The very delicate line of 

 darkness formed by the points of contact between two adjacent 

 molecules, and the tracery-shading produced by their curvature, offer 

 some of the most severe trials of the delicacy of development pos- 

 sessed both by the eye and the instrument. Here we are landed as 

 it were in a fairy shadow land. Too much hght overwhelms the 

 eye and obscures these delicate forms. The least practicable and a 

 bluish light is the best. Sir W. Herschel recommended a quarter 

 of an hour's absolute darkness before submitting the eye to view the 

 faintest objects in the heavens: this rule is too much violated by 

 the microscopic student. A dark room, a shaded eye, calmness, and 

 above all, absence of prejudice, are essentials to microscopic research, 

 not always attainable. 



3Ir. Slaclcs Silica Beads. — A new form of test is here given. 

 On some of his slides isolated silica spherules are copiously visible. 

 Singly observed, there is a bright halo round them with the best 

 glasses ; when two happen to touch, their distinct individuality is 

 obscurely defined. If they were as small as Podura molecules or 

 beads, and placed accurately in a straight row or rouleau, their reso- 



* I left these -with Mr. Cinteis, -who kindly fitted them up, and Mr. Hennah, 

 of Brijjhtoii, subsequently photographed some interesting eflects, which he did 

 me the honour to present to me. 



