PODOSPORIUM. 



[ 611 ] POLARIZATION OF LIGHT. 



Three other British species. 



BIBL. Smith, Br. Dial. i. 82; Kiitz. 

 Batill. 119; Rabenhorst, Alg. i. 110. 



PODOSPO'RIUM, Lev.,= Mdampsora. 

 See UBEDIXEI. 



PODOSTOMA, Clap, et Lach. A genus 

 of Amcebsea (RHIZOPODA). 



Char. Bodies amoeboid, with 2 kinds of 

 pseudopodia, one the ordinary, the others 

 terminating in a flagellum. 



P.JUigerum, among aquatic plants. 



BIBL. Clap, et Lach. Inf. 441 : Kent. 

 Inf. 224. 



PODU'RA, L. A genus of Insects, order 

 Thysanura, family Podurellse. 



The original genus has been greatly sub- 

 divided. In its extended signification, the 

 characters consist in the thorax being di- 

 stinct from the 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. 



Fig. 591. 



Pod ura. 

 Magnified about 15 diameters. 



The body is covered with scales (PI. 1. 

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

 Those of the so-called P. plumbea, which is 

 now described as a Lepidocyrtus, are usually 

 recommended ; but the most common Po- 

 dura is not this species. This, however, is 

 a matter of little importance, because the 

 scales of several species, belonging to even 

 different genera, are similar both in form 

 and markings. 



See SCALES OF INSECTS and TEST- 

 OBJECTS. 



BIBL. Gervais, Walckenaer's Aptires, iii. 

 and the Bibl ; M'lntire, Mn. Mic. Jn. i. 



203 ; 1870, 1 ; Beck, Mic. Tr. 1862, 84 ; 

 Lubbock, Thysanura, Ray Soc. 



PODURHIP'PUS, Megn. A genus of 

 Thysanura, fam. Podurellse. P. pityriasicus ; 

 body fusiform, black, legs paler ; producing 

 pruricro in horses. (Megnin, Paras. 104.) 



PLEUROCHILID'IUM,Stein. A genus 

 of Holotrichous Infusoria. Free, no cara- 

 pace, subreniforni, mouth with a narrow 

 undulating membrane. P. strigilatum, 

 among Lemnce. (Kent, Inf. 540.) 



POLARTSCOPE. A term employed to 

 designate a polarizing apparatus, consisting 

 of a polarizer and analyzer. See INTRO- 

 DUCTION, p. xx. 



POLARIZATION OF LIGHT. The 

 phenomena exhibited by microscopic ob- 

 jects, when viewed by polarized light, are 

 perhaps the most beautiful and interesting 

 of those connected with the use of the 

 microscope. The extreme brilliancy, trans- 

 parency, and variety of the colours deve- 

 loped cannot be equalled ; much less can 

 they be represented by illustrations, although 

 the figures in PL 39 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 be- 

 neath the object and the analyzer above it, 

 the polarizer and analyzer usually consisting 

 of two Nicol's prisms, although two plates 

 of tourmaline are sometimes used. Some 

 artificially prepared crystals exert a power- 

 ful polarizing action, and may be used either 

 as polarizers or analyzers, or as both ; among 

 these the salt of QUININE called Herapa- 

 thite occupies the first place. Others form 

 interesting analyzers, some of which have 

 been noticed under ANALYTIC CRYSTALS 

 and DICHROISII. 



Numerous salts and other crystalline 

 bodies, which powerfully depolarize the 

 already polarized light, and exhibit beauti- 

 ful colours, are mentioned under their re- 

 spective heads ; some of these may be enu- 

 merated here as the oxalate of ammonia, 

 of soda, and of chromium and ammonia, 

 the oxalurate of ammonia, the acetate of 

 copper, chlorate of potash, the prismatic 

 form of the ammonio-phosphate of mag- 

 nesia, the ammonio-phosphate of soda, the 

 sulphates of cadmium and of magnesia, 

 selenite, salicine, fused santonine, uric acid, 

 &c. 



Many animal bodies and tissues also 

 possess considerable depolarizing power, and 

 2n 2 



