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SCIENCE PROGRESS 



microscope, but this can be allowed for. Knowing the width 

 and depth of the field, the volume can be determined. 



This apparatus of Siedentopf and Szigmondy has the 

 advantage that objectives of immersion can be used, since 

 the fulfilment of the necessary conditions is unaffected if a 

 drop of liquid is run in between the object-glass and the 

 upper surface of the preparation. 



Another arrangement satisfying the above conditions, though 

 by a different device, which is especially suitable for the 

 examination of liquids, is that due to Cotton and Mouton. 

 The principle of the method can be readily seen from fig. 3. 

 A convergent beam of light is incident on a block of glass, F — 

 a Fresnel rhomb serves admirably for the purpose. After 

 reflection at the lower surface of the block, the light is brought 



Fitf. 3. 



to a focus in the preparation C, and the angle ot incidence 

 is adjusted so that this light is then totally reflected at the 

 upper surface of the coverslip. Thus the only light entering 

 the observing microscope D will be light diffracted by any 

 particles which break the homogeneity of the preparation. 



Since, for greater efficiency, it is advantageous to be able 

 to use all the light entering F, and to allow none of this to 

 enter the microscope D, the angle of incidence on the upper 

 surface of F must lie between the critical angle for a water- 

 glass surface and that for a glass-air surface— that is between 

 6i° and 41 ; hence a cone of rays, the angle of which is about 

 20°, can be used. The inclination of the surface LM is usually 

 such that normal incidence can be used, and in this way very 

 little light is lost by reflection. 



This form of apparatus is very easily set up ; and a Nernst 

 lamp serves quite well, as a source, for most purposes. It has 



