Ultramicroscopy 



DESIGN AND OPERATION. See OPTICAL THEORY 

 OF THE LIGHT MICROSCOPE, p. 451. 



Ultrasonic absorption microscope 



The desire to explore the world of small Consideration of the propagation charac- 



scale structure (e.g., biological systems) has teristics of acoustical energy in tissue or sus- 



furnished at least a portion of the incentive pensions of biological material suggested the 



for the development of such devices as the development of an instrument which can be 



light microscope and the electron micro- expected to yield information on the struc- 



scope. In each case much new information ture of biological systems not obtainable 



on the microstructure of the system or mate- from either light or electron microscopy 



rial studied has resulted. studies (2, 3). This follows from the fact 



The light microscope (absorption type) that the interaction of the sound waves with 



exhibits certain structural features of cellu- the tissue structure is of a different nature 



lar and subcellular biological organization from that of light or electrons. Experimental 



by means of the contrasting pattern of light results (4, 5, 6) indicate that the protein 



and shade resulting from the absorption of constituents of tissue are largely responsible 



electromagnetic energy to different extents for the magnitude of the absorption of acous- 



by various parts of the structure under tic energy in the ultrasonic frequency range. 



examination. The use of various selective This work also shows that some different 



staining materials such as dyes, which dis- types of protein molecules, at equal concen- 



play different affinity for acidic and basic trations, absorb sound at different rates (7). 



parts of the tissue structure, permits other Therefore, a suitably designed acoustic in- 



aspects of structure to be detected and also strument could yield information on both 



raises the possibility of correlating mor- spatial distributions and identification of 



phology, not directly observable in unstained types of protein in tissue. Since this device 



material, with chemical properties. With the jg ^^ly in the early stages of development, 



advent of phase contrast microscopy it be- ^his article is concerned with outlining the 



came possible, without an increase in resolv- principles of operation, briefly describing 



iiig power, to Identify structural features not ^^^^^^^ ^^ measurements on filament models 



observable with the light microscope em- ,, i,. j-j-j.- j.u u 



, . , , . * . . , ^ at low resolution, and indicating the results 



ploying the absorption principle. » ,, ^. , , i ^- r ^^ • ui 



TTT-,r .1 • 1 r 1 . • of theoretical calculations of attainable 



With the arrival of electron microscopy, 



new details of cellular structure could be ' . . , . 



detected because of the higher available re- ^he results of an approximation analysis 



solving power and suitable impregnation ^^ ^^^ resolving power of this device are 



methods for producing contrast. These selec- g^^en below. The details are included m 



tive impregnation methods result in the reference (3) in which formulas are derived 



retention of salts of heavy metals at certain permitting a calculation of the maximum 



sites in the structure and thus produce con- resolving power attainable with present 



trast in electron transmission because the technology. It should be noted here that 



scattering power of the elements increase as considerable additional knowledge of tissue 



the atomic number becomes larger (1). structure can be expected from an examina- 



544 



