PODOPHRYA. 



[ 514 ] 



POLARIZATION. 



No markings visible under ordinary illu- 

 mination. 



P. jamaicensis (PI. 13. fig. 16). Stalk 

 elongate, weak. Diameter 1-840". 



BiBL. Kiitzing, Bacill. p. 51 ; Sp. Alg. 

 p. 26. 



PODOPHRYA, Ehr.— A genus of Infu- 

 soria, of the family Acinetina. 



P. jixa (PI. 23. fig. 5) is noticed under 

 Actinophrys pedicellata (p. 12). 



It is doubtful whether this is a distinct 

 organism, or whether it is not a stage of 

 metamorphosis of Vorticella. Compare PL 

 25. fig. 33. 



BiBL. Ehrenberg, Infus. p. 305; Dujardin, 

 Infus. p. 266 ; Stein, Infus., passim. 



PODOSIRA, Ehr.— A genus of Diato- 

 macese. 



Char. Frustules concatenate, with a lateral 

 stalk ; valves circular, convex. Marine. 



No markings visible by ordinary illumina- 

 tion. Stalk attached to the centre of the 

 valves. 



P. hormoides (PI. 14. fig. 34). Frustules 

 depressed-spheroidal, connected by isthmi 

 (stalks). Diameter 1-650". 



P. Montagnei {Melosira globifera, Ralfs). 

 Frustules elliptical in front view (circular, 

 R.), connected by short isthmi. Diameter 

 1-600". 



BiBL. Kiitzing, Sp. Alg. p. 26. 



PODOSPHENIA, Ehr.— A genus of 

 Diatomaceaj. 



Char. Frustules attached, sessile, wedge- 

 shaped in front view ; ends indented so as 

 to produce a black line (vitta) in the front 

 view ; valves convex, obovate, with a longi- 

 tudinal median line and transverse striae, but 

 no nodules. Marine. 



The stride consist of rows of dots, some- 

 times distinct by ordinary illumination, at 

 others not so. 



P. Ehrenhergii (PI, 13. fig. 17). Frustules 

 in front view, truncate at the end; valves 

 somewhat acute at the ends. Length 

 1-240". 



P. Lyngbyei. Frustules in front view, 

 truncate at the end ; valves rounded at the 

 free end. Length 1-350". 



Three other British species. 



BiBL. Smith, Brit. Diat. i. p. 82 ; 

 Kiitzing, Bacill. p. 119; Sp. Alg. p. 110. 



PODOSPORIUM, Lev. = Melampsora. 

 See Uredo. 



PODURA, L. — A genus of Insects, of the 

 order Thysanura, and family Podurellse. 



This genus has been greatly subdivided. 

 In its extended signification, the characters 



consist in the thorax being distinct from the 



Fig. 596. 



Podura. 

 Magnified about 15 diameters. 



abdomen, and in the presence of a forked 

 tail, bent under the abdomen when not in 

 use, and enabling the animals to move by 

 springing or jumping, whence the common 

 name of spring-tails applied to them. 



They are of a leaden appearance, and 

 found in shady damp places, as under flower- 

 pots or stones, in cellars, &c., and are about 

 1-20 to 1-10" in length. They may be 

 caught by placing a little flour upon a piece 

 of paper in their haunts. 



The body is covered with scales (PI. 1. 

 fig. 12), which are used as test-objects. 

 Those of P. plumhea, the so-called common 

 spring-tail, are usually recommended; but 

 we believe that the most common Podura is 

 not this species. This is, however, a matter 

 of little importance, because the scales of 

 several species, belonging to even difi'erent 

 genera, are exactly similar, both in form and 

 markings. 



See Scales of Insects and Test- 

 Objects. 



BiBL. Gervais, Walckenaer's Apteres, iii. 

 and the Bihl. therein. 



POLARISCOPE.— A term employed to 

 designate a polarizing apparatus, consisting 

 of a polarizer and analyser. See Intro- 

 duction, p. xviii. 



POLARIZATION OF LIGHT. — The 

 phsenomena exhibited by microscopic objects, 

 when viewed by polarized light, are perhaps 

 the most beautiful and interesting of those 

 connected with the use of the microscope. 

 The extreme brilliancy, transparency and 

 variety in the colours developed cannot be 

 equalled, much less can they be represented 

 by illustrations, although the figures in PI. 

 31 may give some idea of the manner in 

 which they are arranged in certain objects. 



The ordinary arrangement of the parts of 

 the polarizing apparatus scarcely needs de- 

 scription ; the polarizer being placed beneath 

 the object and the analyser above it ; the 

 polarizer and analyser usually consisting of 



