122 MICROSCOPIC TECHNIQUES 



drop. The size of the image is 0.12 X 0.12 mm. The slit image is pro- 

 jected over the opening of the measm-ing photocell (L, Fig. 12) by 

 the objective and the prism (7). The sector is started; the shutter in 

 front of the photocell is opened; and the photocurrent is compen- 

 sated by a potential applied by means of the potentiometer ( U, Fig. 

 13) to the grid connected to the other photocell so that the zero read- 

 ing on the galvanometer may be obtained. The galvanometer is 

 usually constant within 0.5 mm. after 1-2 min. The sample slide is 

 now shifted so that the light will pass through solvent alone or a 

 suitable blank. The sector is adjusted until the galvanometer zero 

 reading is again obtained. It is well to repeat the measurements 

 several times. 



When the mercury lamp is used as the source of radiation, the 

 photocurrent from the measuring photocell is compensated by the 

 photocurrent from the other cell which is illuminated by the semi- 

 reflecting glass as previously mentioned. Thus, with the mercury 

 lamp the compensation current from the potentiometer is replaced by 

 the compensating photocurrent. 



The Sample Slide for Absorption Measurements 



Very clean microscope slides and cover glasses are coated with 

 hydrophobic films of nitrocellulose in order to prevent the aqueous 

 drops from spreading. This is accomplished by pouring over the glass 

 a solution of 1 g. of highly nitrated cellulose (about 13% nitrogen), 

 0.1 g. diethyl phthalate, and 0.01 g. butyl stearate in 100 ml. butyl 

 acetate. The glasses are set aside in a tilted position to drain and 

 dry for at least 3 days in a dust-free place. A drying drum with an 

 electric fan may be used to reduce the drying time. If plastic plates 

 are substituted for the glass, no film is required, but care must be 

 taken that the plastic is optically homogeneous. Two narrow strips 

 of polished glass having a thickness of about 0.35 mm. ( from a hemo- 

 cytometer cover glass) are placed on the hydrophobic film parallel 

 to one another. Between them, sample and blank drops of the order 

 of 0.5-1.0 ju,l. are pipetted, and paraffin oil is added to fill the area 

 between the glass strips. A cover glass is set on the glass strips to 

 complete the cuvette. To determine the layer thickness in the 

 cuvette: 



