NOTES FOR BEGINNERS IN MICROSCOPY. 



149 



The vial may now be slipped under the rubber band into the 

 groove ; the rubber will then hold the vial in place. Turn the 

 vial until the thread in it is on the upper side ; then place the 

 block on the stage of the microscope ; use about | or i inch 

 objective; naturally, sub-stage illumination is used. The light 

 may now be thrown on the thread ; adjust for the thread above 

 the vinegar ; the eels will then be seen going up and down the 

 thread, but cannot get out of focus, although they may, of course, 

 move out of the field. With vinegar well supplied with eels, large 

 numbers of the eels will be in the field at any time of day or 

 night for months. The cork of the vial must have a notch or slot 

 cut in it to admit air to the eels. 



Fig. 7. 



With the mirror swung well to one side and the ink-black 

 background, the eels will appear nearly white. They will often 

 gather in bunches on the thread. This arrangement is very satis- 

 factory, and is very much better than that of placing a drop of 

 vinegar on a slide. If this is done, a i-inch or 2/3rd inch objec- 

 tive is best for their purpose. 



A little more trouble will considerably improve the fixture, but 

 makes it more delicate. The improvement is in cementing with 

 Canada balsam a half-inch or five-eighth inch cover-glass on to the 

 vial, to give a flat instead of a round surface to look through. 



The cover-glass may be protected and the vial held by a thin 

 metal holder instead of the wooden one, the metal holder passing 

 over the vial and cover-glass. 



