air bubble, focus up with central light. The bright 

 spot in the center will become clearer while the edge will 

 become darker. With oblique light the bright spot will 

 be thrown to one side. In studying water, blood or any 

 fluid, always cover the drop with a cover glass. The 

 objectives are corrected for rays passing through media 

 with parallel surfaces. If such a mount is not kept 

 horizontal, currents will be set up, due to gravitation, 

 and they will be seen with a magnified velocity seemingly 

 running up hill. 



The fact that the microscope reverses every move- 

 ment and magnifies it may be mentioned again. 



Beside any movement due to currents there is some- 

 times a peculiar indefinite to and fro movement of par- 

 ticles from one position to another. This is called Brown- 

 ian movement. 



In studying sections a true idea of the structure of 

 the tissue can only be obtained by moving the slide 

 about to bring different parts into the optical axis and 

 by focusing with the fine adjustment to bring different 

 levels, or optical planes, successively into view. Where 

 serial sections are used each section must be studied 

 in relation to its neighbors. 



Sometimes sections which are freshly mounted in 

 balsam appear cloudy and indistinct. This is because 

 of failure to thoroughly dehydrate the specimen before 

 putting it into the balsam. But this brings us into the 

 realm of laboratory technique which is beyond the scope 

 of this little volume, 



