226 



PROGRESS IN MICROSCOPY 



not wet. The portion of their body in contact with the water flattens 

 down the surface of the hquid and the contact angles so formed are 

 measurable. The flat-tints method originates vivid colours in the areas 

 where the surface of the water is flattened down by the insect's contact. 

 Figure 8.19 shows a "Velia" about 7 mm long. The water-surface 

 distortions are observed using the fringes method. 



4. MEASURING DRY MASSES BY MEANS OF AN INTERFERENCE 



MICROSCOPE 



R. Barer, H. G. Davies and M. H. F. Wilkins have suggested 

 applying interference methods to the measurement of living-cell dry 

 masses. Let us consider a diagrammatic cell consisting of a smaU 

 plate of thickness e and surface s (Fig. 8.20). The cell is of a soHd 



(2) 



i 



y-y. 



Fig. 8.20. Diagrammatic cell. 



substance of total dry mass M and a liquid substance of index /;'. 

 The cell is also surrounded by the selfsame liquid. Where living cells 

 are involved, the index /;' selected may well be that of water (1.33). 

 The refraction index of the cell, different from //', equates /;. Let us 

 denote the cell's concentration, i.e. the number of grammes of solid 

 substance it holds per cm\ then, the dry mass of the cell is: 



Let us write: 



M 



K = 



cse 



n — n 



(8.9) 

 (8.10) 



The value of K is 00018 for proteins, 00017 for lipoprotein and 

 0017-00020 for the two types of nucleic acid. Hence in measuring 

 ihe dry mass of cells and other tissue elements composed mainly of 

 these substances the value 0018 may be selected for K (errors wiU not 

 exceed about 10 percent). A^ is a constant for a specific substance. 



