37 



The need for the eareful adjustment of these parts after each observa- 

 tion renders the cardioid instrument troublesome for the rapid 

 quantitative examination of a series of sols. This has been recognised 

 by Siedentopf" who describes an accessory objective for preUminary 

 qualitative investigations. Jentzsch-^ has designed an ultra-condenser 

 specially suitable for the purpose, the cell of which is provided with 

 inlet and outlet for hydrosols, and if desired with electrical and other 

 fittings. Mention should also be made of the Leitz-Ignatowski-' 

 Universal condenser, which may be used for both ordinary and dark 

 ground illumination, but the dark field is not so perfect as in the 

 Jentzsch condenser. 



The determination of the size of metal hydrosols less than 

 5/i/x diameter is best made by the " nuclei " method. It was found 

 that very smaU particles, 5/x/a or less (amicrons), which cannot be 

 counted in the sUt ultra-microscope, but which give to the field a 

 uniform illumination, can be seen by means of the cardioid condenser. 

 Eor the determination of their size an indirect method due to 

 Zsigmondy^^ jg recommended. It is known that a gold sol prepared 

 under standard conditions by reduction with phosphorus, gives a 

 colloidal solution which contains amicrons only — so small as to be 

 beyond the range of the " slit " ultra-microscope. If some of this 

 solution (containing 5 mgs. of gold per 100 c.c.) be added to a 

 colloidal gold solution which is being prepared by other methods, 

 e.g., formaldehyde or hydrazin) the small amicrons function as 

 nuclei around which the gold present in the second solution builds 

 up, forming a larger particle. Doerinckel and Menz-" observed that 

 the size of the particles in the resulting solutions depend upon the 

 number of " nuclei " added. Thus, kno^\ing the amount of gold in 

 the nuclear solution it is possible to calculate the size of the amicron 

 nuclei. Good agreement was found within the range of the sUt 

 ultra-microscope and the method has been frequently used by 

 Svedberg and Zsigmondy to determine the size of amicrons. Recently 

 Zsigmondy has succeeded in increasing the intensity of the orthogonal 

 illumination in a system similar to the " slit " ultra-microscope so 

 that particles between 5ju/x and 3^ju. may now be counted as sub- 

 microns. The diameters of such particles as calculated by the " nuclei " 

 method, from observations in the sHt ultra-microscope, and as directly 

 observed in the new immersion ultra-microscope agree. 



6. Im'proveA Ultra-Microscopy . — Other conditions being equal 

 the brightness of the particles seen in the ultra-microscope depends 

 upon the product of the square of the numerical aperture of the 

 condenser and objective. In the slit ultra-microscope it was found 

 necessarj'^^ to use a condenser of small aperture (-30) and objective 

 of -75 NA. By using a condenser and objective of high numerical 

 aperture, 1-05 N.A. set orthogonally, Zsigmondy^i has very con- 

 siderably increased the intensity of the illumination and the corres- 

 ponding intensity of the scattered light, so that smaller particles can 

 be detected and measured. Since the focal distance of such a lens 

 system is only 6 mm., it was found necessary to cut away a portion 

 of the front lens and mounting, in order to be able to bring both 



