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DEI.L SYSTEM TECHNICAL JOURNAL 



the particles are in constant motion, but when solids or semi-solids 

 are being examined it may be desirable to do so. The mechanical 

 stage of the microscope on which is mounted the adjustable specimen 

 stage allows any layer in the specimen to be brought into accurate 

 focus and hence various strata of the specimen can be examined one 



Fig. 24 — Cartlioid ultra- microscope. 



after llic other. As prc\iously stated, tlie specimen must 1)C pro- 

 vided with two polished surfaces at right angles to each other to 

 correspond to the quartz windows of the Biitz cell. 



Since the observation of ultra-microscopic particles in polarized 

 light supplies useful information respecting their form and color, a 

 polarizer is provided with a hinged stand so that it may be swung out 

 of the optical train. The analyser, as previously mentioned, is 

 fitted over the eyepiece of the microscope. 



The cardioid ultra-microscope illustrated in Fig. 24 differs only 

 in two important features from the ordinary form of microscope. 

 The illumination of the fluid under examination is obtained by a 

 dark-ground condenser njounted in the sub-stage condenser collar 

 and to which Zeiss has given the distinctive name "cardioid con- 

 denser." A diagram of the condenser and the paths taken by the 

 rays is illustrated in Fig. 25. Since the aperture of the rays brought 

 to a focus by the condenser exceeds 1.0, it follows that no light can 

 emerge from the condenser if there be a stratum of air above the 

 condenser. It is therefore necessary to connect the object slidi- 



