I'KOJKCTION MICROSCOPY 



specimens which fluoresce under x-ray radia- 

 tion ; (c) ehminate the effect of electron emis- 

 sion and x-ray fluorescent radiation on the 

 recording material when using hard radia- 

 tion; (d) study specimens which may influ- 

 ence the properties of the photographic 

 emulsion. 



The specimen lies very close to the target 

 of the projection microscope and great care 

 must be taken to avoid reaction between 

 specimen and target. The author notices, 

 for example, that a Mercurichrome-stained 

 specimen may give trouble, if it is used in 

 connection with an aluminum target. 



(4) To reduce spherical and chromatic 

 errors, the lens must have a short focal 

 length (10, 11). As both the specimen and 

 target lie almost in the focal plane of the 

 lens, the specimen is in the active region of 

 the lens field. If the specimen consists of 

 ferromagnetic material it may introduce lens 

 errors and change the focal condition. If the 

 specimen is symmetrically aligned with re- 

 spect to the optical axis and focusing is per- 

 formed with the specimen in position, these 

 errors can be minimized. Experiments, per- 

 formed with the commercial T.P.D. micro- 

 scope show that thin magnetic tape does not 

 appreciably change the focal condition. 



Limitations: The Intensity Problem. As 

 a microscope is aimed to reveal small details, 

 it must have a good resolution. The resolu- 

 tion, however, is among other things propor- 

 tional to the image contrast (12), so the use 

 of long wavelength radiation is imperative 

 in connection with low absorbing specimen. 

 Both the source size and the anode voltage 

 are limited in their lowest values. This is 

 caused by the relatively low specific emission 

 of the cathode, resulting in a very low cur- 

 rent density of the electron spot at the tar- 

 get. Although it may be possible in principle 

 to obtain an electron focus of less than 100 

 A with a 1 kV anode voltage, this is not 

 practicable because of the low intensity. 



The current density of the electron spot 

 is directly proportional to the brightness of 



the electron source and the square of the 

 aperture of the objective lens. The latter is 

 determined by lens errors, especially the 

 spherical aberration. Calculations show that 

 for white radiation, the x-ray energy flux at 

 screen level is: 



p, a JVH»i^C,-^'%-^ 



(1) 



in which ./ is the specific emission of the 

 cathode; V, the anode voltage; d, the diam- 

 eter of the electron spot; Cs , the spherical 

 aberration constant, and h, the target to 

 screen distance. 



Focusing of the electron spot. The energy 

 flux px determines the brightness of the 

 fluorescent image and the exposure time of 

 the film. At the expense of this figure, either 

 the anode voltage V or the source size d can 

 be decreased. The limit will be determined 

 by the impossibility of visual focusing on the 

 fluorescent image rather than by excessively 

 long exposure. An image intensifier will not 

 give considerable improvement as the con- 

 trast and thus the visibility of detail will be 

 limited by the number of x-ray cjuanta used 

 for building an image element. This niunber 

 is proportional among other things to the 

 x-ray energy flux at screen level and the 

 storage time of the eye or screen. By de- 

 creasing the target-to-screen distance b, the 

 brightness of the fluorescent image can be 

 increased considerably at the expense of the 

 field during focusing. The limited field of 

 view is not serious in this case; as in fact, 

 for focusing, one image element is sufficient. 



The condition that the magnification on 

 the screen must be sufficiently high to insure 

 proper focusing sets a limit on the smallest 

 screen-to-target distance. A better approach 

 to the "focusing on one image element" 

 method is realized by Ong and Le Poole 

 (14). Instead of x-raySj they used the elec- 

 trons which are reflected from the target. 

 The reflection coefficient of the order of 2 

 to 5% is greater than the x-ray efficiency. 

 The electrons which have the same energy 

 as the incident ones pass the lenses in oppo- 



663 



