114 



ELEMENTARY CHEMICAL MICROSCOPY 



edge b, the beam of light must be lowered below b. If this 

 is done, the beam of light R will lie too low to be focused, even 

 if the lower lens of the objective is brought into actual contact 



with the upper surface of the object. 

 In this case the beam lies beyond 

 the working distance of the objec- 

 tive. Should we attempt to bring 

 R within the range W, as indicated 

 in the lowest diagram, diffraction, 

 refractions, reflections and disper- 

 sions take place of such characters 

 and to such degrees as to render 

 the detection of micellae impossible. 

 No suggestions as to optical com- 

 binations or size and intensity of 

 the illuminating light beam may be 

 given which will be applicable to all 

 materials. As in all other cases 

 of microscopic investigation, the 

 proper conditions must be experi- 

 mentally ascertained for each prep- 

 aration examined, but it is a safe 

 rule to always avoid too large a 

 slit and too high a magnification. 



For the slit ultramicroscope as 

 made by Zeiss two objectives are 



FIG. 56. The Necessity of having . .... 



Two Sides at Right Angles in the specially constructed, a dry 7 milh- 

 Object for Ultramicroscopic Study, meter, 0.4 N. A. achromatic objec- 

 tive for the study of solids, and a 



4.4 millimeter water immersion of 0.75 N.A., for use with cells 

 containing solutions. A good general outfit should include 

 oculars, i, 6, 8, 12 and 18. 



When polarized light is necessary in the study of colloidal 

 reactions l a nicol prism as polarizer mounted upon a saddle 

 stand is placed between the lens Ci and the slit S. The ana- 



1 For a discussion and explanation of the behavior of colloidal particles in polar- 

 ized light see: Garnett, Trans. Roy. Soc. Lond. (A) 203 (1904) 385. 



