Sept. 15, 1915 Soil Protozoa 513 



in several of the large fields were counted, and the average of these 

 figures was multiplied by 25. The number obtained was then multiplied 

 by the standard number of the platinum loop. With the average of 

 several of such counts and calculations, the number of organisms was 

 determined. In case the organisms were too small to be readily seen by 

 means of the low power of the microscope and too numerous to be calcu- 

 lated from the large divisions of the slide, the approximate number 

 could be obtained by counting the organisms in the smaller divisions. 



When many very active flagellates were studied, the latter method of 

 counting the organisms in several fields proved superior to the complete 

 counting method, on account of the fact that the manipulator might 

 count one organism several times. But, as a general rule, in every case 

 where the organisms were not too active and where the organisms num- 

 bered from 300 to 400 in the solution transferred by the large loop and 

 300 by the small loop the direct count of all the organisms transferred by 

 the loop was made, in order that the error incurred be the minimum. 



In computing the organisms on the slide several fields in different 

 positions were counted and averaged, in this way eliminating as much 

 as possible the very small error which might be incurred, owing to the 

 very slight capillarity. 



The flagellates were always counted while active, as otherwise in many 

 cases they could not be distinguished from cysts. In case of the ciliates, 

 however, the organisms could be very easily distinguished when inactive. 

 Thus, in counting the ciliates, the film of solution was first passed through 

 the fumes of a i per cent osmic-acid solution, as suggested by Goodey (7). 

 When many ciliates were present in a solution containing numerous 

 flagellates, the counts of the living flagellates were made first, and then 

 several other films of solution treated with osmic acid were examined and 

 the number of ciliates determined. 



In order to determine the variation in the amount of solutions which 

 a platinum loop would transfer, two loops of different size prepared for 

 this purpose were tested by weighing films of different culture solutions. 

 The weights recorded below were made by placing the film of the solution 

 upon a watch glass, which was counterpoised by another of the same size. 

 The rider was placed at o.i mg. and the correct addition of weight calcu- 

 lated by oscillations. The temperature of the room was 22° C. and of 

 the balance case 22.5° when the weights were made. (See Tables I-V.) 



