388 JAMES HILLIER 



extremely small unless the instrument is provided with a very large 

 final viewing screen and photographic chamber. Since the intensity 

 of the image varies inversely as the square of the magnification, it is 

 exceedingly low at very high magnifications for illumination inten- 

 sities the specimen can tolerate. This results in poor visibility for 

 visual observation and extremely long exposures for photographic 

 work. In the case of magnetic instruments, the latter requires the 

 provision of power supplies that remain sufficiently constant over 

 such long periods. For these reasons and other less important ones, 

 the magnification obtained electronically is usually kept at the 

 loivest value consistent with the retention in the final micrograph of 

 the resolving power provided by the objective lens. Practical in- 

 strumental magnifications in electron microscopes seldom exceed 

 30,000 X and may run as low as 1000 X. Since the finest details of 

 these images cannot be resolved with the unaided eye, it is customary 

 to magnify the visible image optically for focusing the instrument. 

 The photographic recording of the image without loss of details is 

 accomplished by the use of a fine grain photographic emulsion. 

 From the known resolving power of the photographic emulsion and 

 the resolving power desired in the material under investigation, it is 

 a simple matter to calculate the yninimiini practical instrumental 

 magnification. 



Another important difference between light and electron micro- 

 scopes is in their depths of field. As can be seen from Abbe's formula, 

 in the light microscope the angular aperture of the objective is made 

 as large as possible in order to achieve the highest resolving power. 

 However, since the depth of field of a microscope is determined by 

 the expression D = 2f//tan a (D is the depth of field, d the desired 

 resolving power, and a the angular aperture of the objective) it can 

 be seen that the depth of field of the light microscope where a is 

 nearly t/2 radians is extremely shallow. In fact, the depth of field 

 for high power light microscope objectives is usually less than the 

 resolving power of the instrument. This means that all the prep- 

 arations under examination must be very accurately in one plane if 

 the image is to be in focus at all points. Conversely information 

 regarding a "thick" specimen can be obtained with the light micro- 

 scope by manipulating the fine focus control during visual observa- 

 tion. Such information cannot be recorded in a single photographic 

 record, a fact that explains the conclusion that a photomicrograph 



