PHLYCT^NIA. 



[ 593 ] 



PHOTOGRAPHY. 



xiii. 460; Desmazieres, Ann. Sc. Nat. 8. 

 viii. 16. 

 PHLYCTiE'NIA, Kg,— A genus of Dia- 



tomaceae. 



Char. Frustules those of Naviciila, en- 

 closed in gelatinous globular cells (masses?). 

 Marine. 



P. minuta. Cells 1-720 to 1-240" in dia- 

 meter ; length of frustules 1-1200 to 1-600". 



P. marUima (Fri(dulia mar., E.). 



BrBL. Kiitzing, Sp. Al<j. 96 ; Ehrenberg, 

 Infiis. 232. 



'PHLYCTID'ICJM, Br.— A genus of Uni- 

 cellular Algae, allied to Chytridium ; con- 

 sisting of globose or oblong iuoperculate 

 cells, the orillce simple or slightly produced, 

 either rooted or not ; gonidia with a single 

 long cilimn. Several species; in fresh- 

 water Algse, kc. (Rabenhorst, Fl. Alg. 

 iii. 278.) 



PHLYC'TIS, Wallr.— A genus of Pertu- 

 sariei (Lichenaceous Lichens). 2 species, 

 on trees, frequent. (Leightou, Lichen Flora, 

 237.) 



PHO'MA, Fr. — A genus of Sphaeronemei 

 (Stylosporous Fungi). There are nume- 

 rous British species, forming small black or 

 brown pustules upon dead leaves, twigs, kc. 

 Tulasne regards this genus as formed by 

 pycnidiiferous states of Sph^ria. P. uvicola 

 produces the disease called Anthracose on 

 the vine. 



BiBL. Berk. £r. Fl. ii. pt. 2. 285 ; Ann. 

 X. H. vi. 2G3 ; 2. v. 368, xiii. 459 ; Fries, 

 Summa Vey. 421 ; Tulasne, Ann. Sc. Nat. 

 4. V. 115 ; Cornu, Grevillea, vii. 18. 



PHO'XOLITE. See Rocks. 



PHOR^tllDTUM, Kiitz.— An obscure 

 genus of Oscillatoriaceee, the filaments 

 sheathed, forming a membranous layer ; 

 many species. (Rabenhorst, Fl. Aly. ii. 

 115.) 



PHOR'MIUjNI, Forst.— p. ienax is the 

 name of the plant yielding New-Zealand 

 Flax. It is a Monocotyledonous Flowering 

 Plant belonging to the order Liliacese. 



PHOR'ODON, Pass.— A genusof Aphidse. 

 Two species ; on the hop (Aphis hunmli), 

 the sloe, and the plum. (Buckton, Aphides, 

 Rau Soc, i. 165.) 



PHOSPHORESCENCE and PHOTO- 

 GENIC STRUCTURES, see Mio: Diet. 

 3rd edition, 504 ; and the Bibl. therein. 



PHOTOG'RAPHY.— Microscopic ob- 

 jects may be photographed by the ordinary 

 methods, especially by the collodion pro- 

 cess, by arranging the microscope so as to 

 form the optical part of a camera obscura. 



The old solar microscopes are examples of 

 the principle of such an aiiungement. Mi- 

 croscopic cameras have been constructed in 

 which the lens is replaced by a fitting 

 carrying achromatic object-glasses, with 

 the rod bearing the stage and illuminating 

 apparatus, as in the ordinary stands of com- 

 pound microscopes. A simpler plan for 

 those who possess a compound instrument 

 and a camera, is to remove the lens of the 

 latter and introduce into its place the eye- 

 end (with the eyepiece removed) of the 

 compound body, placed in a horizontal posi- 

 tion ; filling up the crevice all round with 

 a piece of black velvet or cloth. Another 

 method (Wenham's) which dispenses with 

 the camera, is to operate in a room darkened 

 by a shutter having an orifice through 

 which the simlight may be reflected by a 

 mirror placed outside, and received either 

 directly or condensed by a bull's-eye, on the 

 object lying on the stage of the microscope 

 placed horizontally, with the eyepiece re- 

 moved ; a screen placed at a suitable dis- 

 tance from the eye-end of the tube receives 

 the image. In operating with this screen, 

 the object should be focused on a sheet of 

 card, and then, the light being shut ofl' by 

 coveiing the eye-end of the tube, a prepared 

 paper or collodion plate be substituted ex- 

 actly in the same place. Means must be 

 used, by a black cloth or similar contrivance, 

 to shut ofi" all side light between the orifice 

 of the shutter and the object-glass. In 

 this last process, it is possible to obtain 

 pictures with different parts of the object 

 not lying in the same plane, by separate 

 focusing, applying pieces of card suitably 

 cut to shut oft' the image at difierent parts 

 as required. With very minute objects and 

 high powers, the achromatic condenser is 

 used, as well as the bull's-eye. 



It is well known that the correction of 

 the objectives for perfect vision is not the 

 best for photographic purposes. With high 

 powers, as the 1-4" object-glass and upwards, 

 the difference may be neglected, but with 

 lower powers, an adjustment is required. 

 Shadbolt finds it sufiicieut to withdraw the 

 object-glass a little by the fine movement, 

 from the object, and gives the following 

 data for Smith and Beck's object-glass : for 

 the 4-10", withdraw the objective 1-1000"; 

 for the 2-3", withdraAV it 1-200"; for the 

 1|", withdraw it 1-150''. Wenham pre- 

 fers to place a doubly convex lens in the 

 place of the back stop of the object-glass, 

 and advises for the 4-10" and 2-3" object- 



2q 



