112 BELL SYSTEM TECHNICAL JOURNAL 



to exclude light of wa\'C-lengths other than that for which the objective 

 has been computed. 



In high-power photomicrography of metallurgical specimens, the 

 purpose, of course, is to attain the maximum of resolution and here 

 the wave-length of the light used plays an important part. As 

 mentioned above the resolving power of an objective may be increased 

 by decreasing the wave-length of the light used. Assuming that a 

 Wratten "F" filter is used whose transmission band is from 0,100 A.U. 

 to the red end of the spectrum, then an objective of 1.4 X.A. should 

 resolve about 109,000 lines per inch. If a "C" filter is used whose 

 spectral transmission is from 4,000 A.U. to 5,100 A.U., the s;ime 

 objecti\e should resolve about 158,000 lines per inch. In other 

 words, by using the shorter wave-length light,' it is possible to effect 

 a theoretical improvement of about 45% in the resolution. In 

 practice, these theoretical values are not fully obtained because of 

 other complications entering into the problem. 



Polarized Light 



Polarized light is oftentimes a very useful aid in the study of trans- 

 parent objects. By combination with suitable selenite plates color 

 comliinations arc de\cloped in the specimen and between the specimen 

 and the background which facilitate identification of substances, 

 comparison of known and unknown substances, and the study of 

 their structure. In the field of crystal studies, polarized light is 

 indispensable and it furnishes evidence of a very substantial nature 

 in the field of micro-chemistry. The problem has been presented on 

 occasions to identify the nature of some substance, resulting from 

 the corrosion of some small telephone part. The evidence in these 

 cases could easily be placed on the head of a pin but by the use of 

 polarized light in conjunction with micro-chemical methods, it has 

 been possible to form some sort of a qualitative estimate of the nature 

 of the substance. Polarized light is obtained by means of a nicol 

 prism contained in a suitable mount which is clamped in a ring beneath 

 the sub-stage condenser. The illuminating beam from the microscope 

 mirror is thus polarized before it reaches the condenser. A second 

 nicol prism called the analyser is either contained within a special 

 eyepiece or the analyser takes the form of a mount which may be 

 placed above the usual eyepiece. Both polarizer and analyser are 



' Rc'KanlinK tliu use of iiUra-violet light sci- Higli-Powcr Photomicrography of 

 MflalhirKical Specimens, !•'. 1". Lucas, Trans. Am. Soc. for Steel Treating; Vol. IV, 

 p. Oil, 1923. 



