Commercial Electron Microscopes 75 



solution which is to be studied is applied to it. Finally, the mesh 

 with the preparate is placed over the opening of one of the small 

 cartridges which can be seen in the foreground, and the cartridge 

 is loaded into the object chamber of the microscope. This is 

 the routine which can be followed, e.g., in the investigation of 

 bacteria and viruses. Other objects require special techniques 

 of which a great number are in existence, and which are growing 

 almost daily. 



The R.C.A. Type B electron microscope has been fitted lately 

 with a number of accessories for special investigations which are 

 embodied in the new "Universal" E.M.U. model, to be de- 

 scribed later. 



2. R.C.A. Magnetic Microscope, Desk Model 



Several years of experience in the application of the R.C.x\. 

 microscope, Type B, enabled the R.C.A. to compile interesting 

 statistics of the most frequent applications.^^ It was found that 

 the greatest interest lies in a range of sizes and details for which 

 even a resolution limit of 20 A is barely sufficient. But the 

 statistics revealed also considerable interest in objects exceeding 

 about 800 A in size where a resolution of about 80-100 A would 

 be all that is required. 



Statistics were made also of the magnifications preferred by 

 research workers in various fields and of the number of expos- 

 ures usually taken of one specimen. It turned out that relatively 

 few workers availed themselves of most of the steps which are 

 provided in the large R.C.A. model between the minimum of 

 1,000 and the maximum of 20,000, and that in the great majority 

 of cases only one photograph was taken of each specimen. These 

 and other considerations convincingly proved the need for a 

 simplified instrument. It was decided, therefore, to develop a 

 small electron microscope with only two magnifications, 500 and 

 5,000, and with only one exposure per plate. An experimental 

 model embodying these and many other distinctive features has 

 been described by Zworykin and Hillier.^^ Though some of its 

 constructional details have been redesigned, most of the func- 



