lyo 



Journal of Agricultural Research 



Vol. XVII, No. 4 



was continued with a mixture of freshly precipitated calcium carbonate 

 and magnesia hydrate until a microscopically smooth and essentially 

 flat surface was obtained. Finally, in the center of the cell a circle i 

 mm. in diameter was inscribed with the diamond-point marker of the 

 microscope in order to limit the field of observation. A diagram of the 

 slide, with2>2 mm. cell (A) containing circular area (B) drawn to natural 

 scale, is shown in figure i , where the depth is indicated as lying between 

 0.044 3-11^ 0-143 mm. These values were determined as accurately as 



r 



-4^ 



^efiMofcell O oaa fo O /43 mm. t 



/J /S saiJCer~5hape<f ce/f 2imm /o cf/ometer 

 O is Gtrcu/or fie Id I mm in diameter in center of eel/ floor. 

 Fig. I. — Glass slide with ultra-microscope cell drawn to natural scale. 



possible for each cell by means of a strain dial recording intervals of 

 o.oooi inch and were checked with the microscope micrometer under a 

 magnification of 740 diameters. A blunted, highly polished needle point 

 inserted in the vertical arm of the dial enabled readings to be taken at 

 different points within the cell, and the average of these readings was 

 compared with that of an equal number taken around the cell from 

 without. The results of these measurements for a number of cells con- 

 structed in the manner outlined above, together with microscopic check 

 determinations, inclosed in parentheses, are given in the following table : 



Table I. — Depth of ultra-microscope cells determined by strain dial 



Slide number. 



Depth in 

 mm. 



Thickness 



of slide in 



mm. 



Maximum 

 variation 

 in depth 

 in mm. 



in TTITTI ^ 



Percentage 



of variation 



in thickness 



of slide. 



I 



2 



3 



4 



5 



6 



Average 



a( 



1209 



1220) 



1321 



0434 



0423) 



1432 



1450) 



0795 



0790) 



0892 



0890) 



1014 



I- 5316 



I- 5382 



I- 7374 



I. 6103 

 I. 4880 

 I. 6812 



I- 5978 



o. 0058 



0160 



0036 



0094 

 0071 

 0043 



• 0077 



o. 0061 



. OIOI 



. 0096 

 . 0046 



■0053 



. 0061 

 . 0070 



12. 10 

 8.30 



6. 50 

 9. 20 



4.82 



7-53 



o. 40 



66 



55 



28 

 36 

 36 



43 



a Determined by microscope micrometer. 



