147 



owing to a want in the faculty of perception, due 

 to a too slight acquaintance with the phenomena. 

 These trials should be repeated until the proper 

 sensitiveness of feeling in making the adjustments 

 is acquired and until they can be made to corres- 

 pond with a certainty to at least within two 

 divisions. When it is found after repeated trials 

 that sufficient skill has been acquired, mark the 

 number upon the slide. For future examination of 

 the same slide, this will facilitate work and give 

 the assurance that the best results are thus ob- 

 tained without further trial' by simply referring to 

 the recorded number. 



On stained Bacteria and Micrococci, focus briskly 

 with the fine adjustment to either side of exact 

 focus. There will be an expansion of the outline 

 of the object both when within and without the 

 focus. If the greater expansion or coma is within 

 the focus, or when it is necessary to raise the 

 objective, there is spherical over-correction and 

 the adjustment must be closed. When the proper 

 point _of correction is reached the expansion of 

 outline is the same in both directions. 



Immersion Objectives. As has been stated 

 before, immersion contact between the objective 

 and cover glass is made by either water or homo- 

 geneous fluid. Distilled water only should be 

 used and kept in a suitable bottle. Cedar oil is 

 used for the homogeneous fluid and is thickened 



