172 THE TECHNICS OF PHASE MICROSCOPY 



may be essential for some specimens when the ilUimination is good. A 

 15 X or IGX ocular is recommended, and spectacle wearers will find 

 high eyepoint oculars comfortable and efficient. 



.3. GENERAL PRINCIPLES FOR APPLIED PHASE MICROSCOPY 

 3.1. Preparation methods 



Many materials may be examined satisfactorily in water or in the 

 liquid normally accompanying them. Such preparations should be 

 quite dilute and, when the oil immersion objective is to be used, c^uite 

 thin. One method places a small drop at the center of the slide of such 

 size that it will spread to about half the area of the cover glass. A little 

 petrolatum jelly, about eciual in size to the shaft of a common pin, is 

 spread along the four edges of the cover slip. The cover glass is inverted 

 over the tiny drop and pressed into good contact by drawing a dissect- 

 ing needle or other smooth small point around the periphery of the 

 glass. This seals the preparation against evaporation and prevents its 

 being dislodged when the preparation is moved during study. A less 

 viscous immersion oil is preferable between the cover glass and the ob- 

 jective. By pressing the cover carefully into the sealing material the 

 movmt may be made as thin as desirable. Convection currents and 

 motion are greatly restricted in such a thin preparation. White 

 petrolatum jelly may be acid from bleaching, and fluids containing 

 living material should not come in contact with it. When petrolatum 

 jelly is not a\'ailable, paraffin oil or Crown immersion oil may be used 

 to seal a temporary preparation. A useful mount is the Rotocompressor 

 of Wicterman (1940), especially an improved type with lower metal 

 sides. Cover glasses 0.18 mm thick are essential for best results, as 

 objectives are corrected to this standard. Other thicknesses decrease 

 contrast, owing to added spherical aberration. 



Hollow-ground cells and hanging drop mounts act also as lenses which 

 prevent focusing the image of the annulus on the diffraction plate and 

 are not suitable for phase microscopy. Hanging a small piece of cover 

 glass on the hanging drop will often flatten it enough to make examina- 

 tion possible. Flat-bottomed depression slides are acceptable when the 

 surface is good enough optically not to interfere with focusing the 

 annulus image on the diffraction plate. If any wedge is present it will 

 change the centration of the annulus as the preparation is moved during 

 examination, and constant recentering with the aid of the telescope 

 will be necessary to avoid loss of contrast. When the wedge effect is 

 marked, it may be compensated for by placing a weak prism, such as 

 a 2- to 5-A spectacle lens, over the condenser. 



