58 ELEMENTARY CHEMICAL MICROSCOPY 



right angles to one another and an illumination device for pro- 

 jecting a tiny beam of light into the cell in a line at right angles 

 to the optic axis of the microscope. The tiny beam of light is 

 obtained by means of small projection lenses and an adjustable 

 slit. To distinguish this type of illumination from others com- 

 monly employed in microscopy, the term " orthogonal illumina- 

 tion" has been proposed. It is obvious that in this system no 

 direct light can enter the objective but only such rays as are 

 diffracted by the particles in suspension in the liquid contained 

 in the cell. 



The form and arrangement of the component parts of the 

 slit ultramicroscope naturally differ according to the optical 

 firm manufacturing the instrument. One of the best known 

 and most frequently used types is that shown in Fig. 24. 1 This 

 instrument consists of an optical bench B, at one end of which 

 is placed an arc lamp R and at the other a compound microscope. 

 Between the lamp and the microscope there are a series of con- 

 densing lenses and an adjustable slit. The light rays emanating 

 from the arc are collected by the spherically and chromatically 

 corrected lens Ci of 80 millimeter focus, so placed as to project a 

 very bright image of the crater of the arc upon the slit S. In 

 ordinary use this slit has its length in a horizontal position, the 

 width being controlled by the micrometer screw with graduated 

 head G, while the length of the slit is regulated by the screw s. 

 After passing through the slit the light rays enter the lens 2, 

 having a focal length of 55 millimeters, whose function is to 

 project a reduced image of the slit into the condenser-objective 

 C 3 . Since both slit and lens 2 are movable forward and back 

 upon the optical bench, the lens C% serves a double purpose, 

 projection and adjustment of the magnitude of the light beam 

 entering Ca. The objective Cs projects into the preparation 

 contained in the cell of black glass U a tiny conical beam of 

 light at right angles to the optic axis of the microscope M. To 

 prevent any side light from entering the preparation, lenses Ci 

 and 2 are small and are mounted in blackened metal screens; 

 as a further precaution a large metal screen D with tubular 



1 Manufactured by Carl Zeiss, Jena. 



