1 68 Journal of Agricultural Research voi. x\ai, no. 4 



where part of the rays were deflected from the surface of the suspended 

 particles into the microscope, thus rendering them self-luminous and 

 clearly visible while the remainder of the field remained dark. A similar 

 effect may be obtained by means of a substage parabaloid condenser, 

 with central stop, whereby the outer rays from the beam of light entering 

 the microscope from below are brought by a series of reflections to a 

 short focus within the cell and are totally reflected from the lower surface 

 of the cell cover, leaving the field dark as before. Particles whose indices 

 of refraction vary from those of the inclosing liquid intercepting these 

 oblique rays diffract a portion of the light into the microscope and be- 

 come luminously visible as in the former case while the remainder of the 

 field is perfectly dark.^ 



In order to avoid loss of light through refraction of the rays issuing 

 from the condenser, an immersion liquid, such as cedar oil or glycerin, 

 should be employed between it and the cell containing the liquid under 

 examination. This parabaloid illuminator is interchangeable with the 

 substage Abbe condensor of the ordinary microscope and was conse- 

 quently found most convenient for the present investigations. The micro- 

 scope selected was provided with an accurately calibrated micrometer 

 screw for vertical measurements and a mechanical stage for lateral 

 orientation. A diamond point object marker with circular movement 

 graduated to milHmeters and insertible in the revolving nosepiece of the 

 microscope will also prove a useful accessory. Light was furnished by 

 a special arc lamp run on either direct or alternating current and regu- 

 lated by a rheostat of 4.5 ampere capacity. Before entering the micro- 

 scope the light was passed through a cooling solution, acting as a ray 

 filter, of 10 mgm. diamine green dissolved in i liter of distilled water. 

 A photograph of the microscope with arc light and ray filter used is 

 shown in Plate 19, A. 



The ordinary lens system of the microscope consisted of eyepieces 

 X 7.5 and X 12.5 and objective 3, 4, and 16 mm., giving linear magnifica- 

 tions of 50 to 740 diameters at a tube length of 160 mm., while the 

 best combination for counting was obtained with eyepiece X7.5 and 

 objective 4 mm., magnifying 320 diameters. 



A counting device was inserted in the focal plane of this eyepiece, 

 consisting of a cross-line micrometer scale with ground glass border 

 divided into 25 square areas each side of which measured 1.25 mm. 

 and corresponded exactly to 0.05 mm. of a stage micrometer at a tube 

 length of 166 mm. With this micrometer the areal dimensions of any 

 liquid under examination could be accurately determined, while the 

 vertical element was obtained by means of the micrometer screw record- 

 ing an interval of 0.00254 mm.^ 



1 Burton, E. F. the physical properties of colloidal solutions, p. 46-47. London and New 

 York. 1 9 16. 



* This micrometer was calibrated against that of a standard Fuess microscope registering a m inim u m 

 vertical interval of o.cxji mm. 



