52 MORPHOLOGIC VARIATION 



bacillus described in Chapter IX, and here the death curves were 

 determined by plate counts. 



The measurements of morphologic characters, size, and form of 

 the cells, were accomplished in various ways. With Bacillus mega- 

 therium, because of its large size, these measurements were made 

 fairly easily. Microscopic preparations were projected, using a 1.5 

 mm. apochromatic objective and a 1 2 X periplan eye piece, with an 

 arc lamp as illuminant, onto a screen at such a distance that the 

 magnification was approximately 3000 X. The individual cells were 

 then easily measured simply by spreading dividers over the pro- 

 jected image and holding these dividers to a ruled scale. In some 

 cases measurements have also been made by the use of the Abbe 

 camera lucida, simply spreading the dividers along the table over 

 the apparent image of the cells. For the smaller organisms, how- 

 ever, it is necessary to obtain a higher magnification, in order to 

 have reasonably accurate results; and since projection at magnifi- 

 cations beyond 3000 X is impractical, because of the difficulty in ob- 

 taining sufficient light, the following procedure was adopted: 



Photomicrographs were prepared of numerous fields, using tne 

 above named lenses, the magnifications varying from 2000X to 

 3000 X, according to the bellows draw, and the photographic nega- 

 tives were again projected at a magnification of seven to ten times. 

 All photographs were made on panchromatic film, using a Wratten 

 red filter, which gave the maximum contrast. The projects images of 

 the cells were then either measured directly, or more frequently they 

 were traced onto individual cards and the drawings on these cards 

 were then measured for area by use of a planimeter, and for length by 

 means of dividers. This last procedure may be considered as involv- 

 ing considerable error, because of the extreme magnification, the final 

 magnification being about 20,000 X. Naturally with such magnifi- 

 cations the edge of the image is far from being sharp, there being 

 a zone of fuzziness corresponding in dimensions to the limits of reso- 

 lution of the microscope, that is, to about 0.15[jl in terms of the 

 original microscopic preparation. This error is, however, a com- 

 pensating one, for in trying to place the dividers or trying to trace 

 with the pencil as far as possible through the center of this zone of 

 unsharpness, the chances are as great for going inside the middle 



