and Laboratory Methods. Uil9 



to focus a low power lens exactly on an object so that it is distinctly seen on the 

 ground glass ; then remove the object and so adjust the light and condensers 

 that an image of the source of light is sharp on the ground glass ; then put on 

 high power objective desired and the instrument is ready for use. Various 

 arrangements of the condensers and light will produce this image ; and the image 

 will vary in size with the several arrangements; some particular arrangement 

 will be found best for each objective, or at least for each group of objectives. 



In doing miscellaneous work it will often become necessary to change from 

 one arrangement to another. For this reason the rods carrying the light and 

 the condensers should be so graduated that permanent records of each arrange- 

 ment can be made. This is of such importance as a time saver that I think it 



Bacillus tetani in both the vegetative and spore forming conditions, x 1500. 



should have the attention of instrument makers. We experimented for months 

 with our instrument to little or no purpose ; this we afterward found arose from 

 the fact that nearly right was entirely wrong. Several evenings would be wasted 

 in getting a correct arrangement ; we could then work rapidly while we required 

 the same arrangement ; but to break it up and return to it again was almost the 

 same trouble it was to find it at first ; our real progress began when we had care- 

 fully graduated both tables so that the Abbe, the double convex lens, the con- 

 denser with its adjustable front and the light, five variables, could be brought 

 quickly into any desired relationship. 



The image of the luminous portion of the light must of course be as large as 

 the required field, or the above arrangement will not do; accordingly, for low 

 power work the image of the light must fall on the objective instead of the object. 



