MEMBRANE -INTERFEROMETER VOLUMETRY 



341 



displacement of the film with respect to the glass slide can be fol- 

 lowed by recording the magnitude of the shift of the interference 

 fringes produced. A movable ocular micrometer may be used to 

 measure this shift. 



Microscope - 



nj 



Mirror 



(D 



Source 



Shellac 



Membrane 



Microscope slide 



Cover slips 



Fig. 109. Membrane inter- 

 ferometer manometer. 

 Froyn Tobias (1942) 



Adjustable 

 diaphragm 



Fig. 110. Detail of membrane inter- 

 ferometer manometer, showing that 

 fringes maj^ be obtained by reflected 

 light by shifting source, diaphragm, 

 and mirror from below to above the 

 interferometer. 



Frum Tohins (19.^2) 



The sensitivity of the instrument may be increased by employing 

 shorter wavelengths of light. Tobias used the green mercury line at 

 546 m/i, after the light was passed through a Corning filter. The use 

 of a diaphragm to narrow the light beam lends sharpness to the 

 fringes. By placing a glass slide over the top cover slip, a differen- 

 tial manometer with closed chambers on each side of the membrane 

 is formed (as in Fig. 110). One of the chambers may be used as a 

 reaction vessel, or, as water may condense on the membrane, it 

 might be preferable to make connection between the chamber under 

 the film and a reaction chamber ground out of the slide a short 

 distance away by cutting out a segment in the cover slip to form a 

 capillary channel between the two. 



Drilling Holes in Cover Slips. A pile of ten to twelve cover 

 slips are mounted in pitch on a heavy glass plate. Round holes are 

 drilled with a carborundum or silica pencil bit operated in a hand 

 drill using water as the lubricant. The pitch is removed by soaking 

 in xylol for a day. The serrated edges are no handicap, but they can 

 be rounded off by coating with several layers of a plastic. 



Preparation of Membranes. The collodion films are made 

 from a mixture containing 25 ml. celloidin {Merck), 25 ml. amyl 



