APPENDIX 517 



2. Using a slide of algal filaments, mounted and covered 

 as before, examine some favorably exposed green filaments, 

 using all the sizes of diaphragm you have available in suc- 

 cession, and finally using no diaphragm at all, observing the 

 while the relative distinctness of the green chlorophyl, and 

 of the clear transparent cell wall. Thus you should learn 

 the advantage of stopping out the excess of light. 



On the use of the higher power lenses. 



1 . In rapid focusing, while finding an object under the 

 compound microscope, never run the objectives downward 

 while looking through the tube : for thus you would soonei 

 or later jam an objective into the cover glass, possibly 

 breaking both. The objectives as well as being the most 

 expensive part of the instrument, are the parts most easily 

 abused. While looking from the side, push the objective 

 down close to the object (close enough to be within the focal 

 distance of the objective, previously determined); then 

 place your eye to the eyepiece and draw up slowly till the 

 object is somewhat visible; then finish focusing with the 

 fine adjustment. 



2. Determine the actual size of the field with the differ- 

 ent combinations of lenses available, by examining a 

 mounted strip of paper printed with fine and regular half- 

 tone stippling (better mounted in glycerine and covered) 

 with them successively, and observing the extent of the 

 stippling included in the field with each. Thus you may 

 determine what combination will be available for the exam- 

 ination of objects of different size. 



3 . Learn once for all how to find an object speedily and 

 certainly under the high power objective; proceed as fol- 

 lows : find the object or the part of it desired, first with the 

 low power objective; then place the part of it to be exam- 

 ined with the high power in the centre of the field, change 



