262 



PROGRESS IN MICROSCOPY 



same result in monochromatic light. The photometer assembly proper 

 is adaptable to a standard microscope. There is a similar type of 

 instrument designed for photometric work with opaque objects. 



The Cooke, Troughton and Simms photometer eyepiece may be 

 used, either with a separate light source, or a single source as in Berek's 



Fig. 10.18. The Cooke, Troughton and Simms photometer eyepiece. 



device. The comparison surface consists of the small metallized 

 surface /?, located at the tip of the glass rod / and cut at 45^. The 

 device for adjusting the luminance of the comparison surface consists 

 of the three polarizers (polaroid) A^^, No, N3. 



The polaroid elements A^i and N^ are fixed and their axes parallel. 

 The polaroid A^2 is rotatable and its orientation shown by a graduated 

 circle. Under such conditions, the transmitted intensity is subjected 

 to a s'm*0 (0 = angle of rotation) instead of the sin-0 law obtained 

 with 2 polarizers. The polaroid element A^, eliminates the errors due 

 to a contingent polarization of the hght source. Monochromatic 

 filters can be added. 



We would also mention a very simple device which we have used 

 (Fig. 10,19). It consists of a plastic-material rod (diameter: about 

 3 mm), which conveys the comparison Hght into the eyepiece. The 



