BY R. GRBIG SMITH. 535 



double printing they obtain an image of the bacteria upon a 

 network of squares. This method is good for recording the 

 measurements in a pictorial manner, and for actually measuring 

 the breadth it is better than the micrometer eyepiece, inasmuch 

 as the unit of size becomes 1 fi instead of 1 -5 or 1 -8 p. For exact 

 measurement, it would be easy, once the bacteria were photo- 

 graphed, to place the negative under a low power objective and 

 measure the breadth with the micrometer eyepiece, or to project 

 the image upon a screen by means of the projection lantern and 

 measure with a centimetre or millimetre rule. This of necessity 

 involves photographing the organism, a process which is not 

 always desired. 



The method I employ in determining the breadth of an 

 organism is to fix upon a bacterium in the microscopic field and 

 measure its length. Then I compare the organism with a series 

 of diagrams representing bacteria, the breadths of which have 

 been accurately measured in terms of the length. From this 

 series one group that appeal's identical with the organism fixed 

 upon is noted, and the number of this group is multiplied l>y the 

 length of the organism. The result is the breadth. The breadth 

 of another organism in the same film may be calculated in a 

 similar manner, and the second result will generally be identical 

 with the first. For example, the organism is a short rod measur- 

 ing 1 -5 fj., and on comparison with the diagrammatic table it 

 appeal's identical with the group whose type number ( "^th' ) * s 

 0-4. On multiplying 1-5/a by 0-4, the breadth 0-6^ is obtained 

 This result will be more exact than that obtained by estimating 

 the breadth with the micrometer eyepiece. Since this estimation 

 of the breadth when done from a longer and a shorter form in 

 the same film necessitates two different calculations, these, when 

 they agree, are more likely to be correct than when several are 

 estimated by a similar mental estimation as obtains when the 

 eyepiece micrometer alone is used. It, however, goes without 

 saying that it is advisable to check the one method against the 

 other. 



