Commercial Electron Microscopes 71 



a handle. The manipulation of this handle lifts up the specimen 

 holder and at the same time operates the airlock, isolating the 

 object chamber from the rest. Air is admitted automatically on 

 unlatching the door which can be seen in the figure, and the 

 specimen holder can be removed. On loading, the same opera- 

 tions are performed automatically in opposite order. 



The photographic plates used in the R.C.A. microscope 

 measure 10 in. by 2 in. and can be used for a series of exposures, 

 usually 2 in. by 2 in. each. Like the specimen, the plate is intro- 

 duced through an air lock, without breaking the vacuum in the 

 main chamber. Reevacuation takes only 2-3 minutes. Fine 

 grain photographic plates are used with preference. The maxi- 

 mum obtainable magnification is about 20,000, which means that 

 the finest observable detail of about 30 A will be magnified to 

 about 0.06 mm. In order to avoid any disturbance by photo- 

 graphic grain this ought to be about ten times the grain size, i.e., 

 the plate should have a resolution of about one hundred fifty 

 lines (black and white) per mm. It would not pay to use plates 

 of higher sensitivity, as the grain size goes up roughly propor- 

 tionally to the sensitivity, and apart from the disadvantage that 

 the microscope would have to be made longer or more com- 

 plicated by adding a third stage, the available light intensity 

 would decrease with the square of the magnification. Therefore, 

 it is much preferable to magnify the photographs by optical en- 

 largement to a size suitable for studying with the unaided eye. 

 A final magnification of 30,000-45,000 is fully sufficient if the 

 enlargements can be studied- in the original, in half-tone repro- 

 ductions as much as 200,000 may be required to bring out every 

 detail clearly. 



The pillar behind the microscope proper contains the high- 

 voltage supply, the circuits to energize the coils, and the stabiliz- 

 ing and controlling equipment. Figure 20 gives an idea of the 

 complication of the auxiliary gear which comprises 53 valves. 

 Only a very cursory description can be attempted here. 



Fluctuations of the driving voltage can be divided into two 

 classes : a-c ripple and variations of the mean value. The latter 

 are mostly slow variations, due to inconstancy of the line voltage 



