PART SECOND. 



STRUCTURAL ELEMENTS, 



PRELIMINARY STUDY. 



FOKM OF OBJECTS. 



From a single and hasty view of bodies under the microscope, we 

 are liable to form erroneous ideas of form. Either a sphere, disc, 

 ellipsoid, ovoid, or cone may be so viewed as to present a circular out- 

 line. It therefore becomes important to view objects in more than, 

 a single position. This can easily be accomplished with isolated 

 particles by suspension in a liquid. In this way the true shape of a 

 blood -corpuscle, e. g., may be determined. 



Again, much information concerning the actual form of bodies may 

 be gained by a proper adjustment of the fine focussing screw. You 

 may remember that the depth of the field of view in the microscope 

 is exceedingly slight. Speaking accurately, only a single plane can be 

 seen with a single focal adjustment; but by gradually raising or low- 

 ering the tube of the microscope, the different parts of a body maybe 

 focussed and studied and an accurate idea of form secured. 



With a glass rod place a drop of milk, which has previously been 

 diluted with three parts of water, on a slide, and put a cover-glass 

 thereon as in Fig. 19. Focus first with the low power (L). A mul- 

 titude of minute dots are observed. Then switch on the high power 

 (H), and the dots will resolve into circular figures. Select one of the 

 smaller particles and, as you raise the focus, the centre of the figure 

 retains its brilliancy, while the edges become dark or blurred, show- 

 ing convexity. Reverse the focus, and the centre again retains its 

 sharpness long after the edge has become blurred. The figure, then, 



