THE OBJECT 100 



objective; and microscopic (or submicroscopic) objects, 

 in (water or) immersion oil (or balsam), suited to the high- 

 power, oil-immersion objective. 



Practical Points 



1. Use preferably the water-iniiiiersion objective for objects 

 in watery fluids which arc more than 10 microns in depth. 



2. Keep the oil-immersion objective mainly for objects in 

 immersion oil or a similar medium; or, if used on objects in water, 

 for those which are in contact with the cover-glass, or less than 10 

 microns distant from it. 



3. For investigation, temporary preparations in watery 

 fluids are sometimes to be preferred to permanent mounts. 



4. Stained bacteria and spirochsetes often show well with 

 dark-field illumination, mounted in air (Coles's method). 



5. Choose an appropriate medium for mounting the prepara- 

 tion for investigation. The optically best media are air, water, 

 immersion oil, and monobromide of naphthalin, for the four 

 classes of objectives, respectively. 



6. With the binocular magnifier and the Greenough, use as 

 thin a water layer as convenient for objects under water. 



7. Keep the main axis of the binocular magnifier or Greenough 

 at right angles to the water in the dissecting dish. 



8. Have plate glass discs if possible to cover Petri dishes when 

 under microscopical observation. 



9. Use rather a too thin than a too thick cover-glass, when a 

 cover of the right thickness is not available. The same is the 

 case with the slide. 



10. It might be worth while in some cases to experiment with 

 bacteria, smears, and submicroscopic organisms mounted on the 

 cover-glass dry (stained or not), to be viewed with oil-immersion 

 objectives through the cover. 



11. To mark objects, draw an ink circle, with a fine pen and 

 waterproof India ink, around the small circle of light on the 

 object; there being a small diaphragm on the source of light. 

 Or, better, draw a small circle around the object on a slip of paper 

 equal to the slide and superposed on it (Bridges). This is after- 

 wards placed on the slide, and the position of the centered image 

 of the diaphragm on iho source made to coincide with the circle 

 drawn on the paper, by shifting slide and paper. The writer 

 uses this method in preference to reading the scales of the detach- 

 able mechanical stap;e. 



