ULTRAMICROSCOPES 121 



carefully laid thereon, avoiding all dust particles and air bubbles. 

 This thin film of oil brings about an optical homogeneity be- 

 tween prism and slide. By means of a condensing lens C of 

 about 15 centimeters focus the rays RRR emitted from an arc 

 lamp as radiant are projected into the prism through the in- 

 clined face, the inclination 6 of this face being approximately 

 51 degrees. These rays are totally reflected and are brought to 

 a focus at the upper surface of the glass cover at the angle of 

 total reflection. Any particles in suspension in the liquid will 

 diffract the light and diffraction disk images will be formed in 

 the microscope. No other light can enter the instrument and 

 we therefore have the theoretical conditions necessary for the 

 demonstration of ultramicroscopic particles, namely, the par- 

 ticles become luminous upon a black background, the illuminat- 

 ing rays being of high aperture while the image-forming rays are 

 of low aperture. 



The adjusting of the illumination in this device consists in 

 ascertaining (a) the proper inclination of the rays entering the 

 prism, and (b) the correct distance of C from P, so that the focal 

 point will fall in the proper plane. This adjustment requires 

 considerable care and should first be undertaken by means of 

 some preparation of a colloidal metal (silver, for example), and 

 after having obtained the optimum conditions in this manner, 

 the preparations to be studied are then substituted for the test 

 object. 



This type of ultramicroscope is applicable only to the examina- 

 tion of liquids. With proper care in adjustment it will yield 

 results fairly comparable with the slit ultramicroscope. 



In many types of investigation this device possesses a very 

 desirable feature, namely, that of permitting at any time an 

 examination of the preparation by ordinary transmitted light, 

 for it is merely necessary to tip the mirror of the micro- 

 scope and thus send rays M through the object in the usual 

 manner. 



Absolutely clean glass surfaces free from scratches and inclu- 

 sions are essential. For cover glasses the use of thin freshly- 

 prepared cleavage films of clear mica is suggested by Cotton. 



