266 PROJECTION MICROSCOPE \CH. IX a 



have been obtained with hematoxylin and eosin, and the various car- 

 mines. Every method of staining which gives either sharply differen- 

 tiated results or transparent colors produces preparations adapted to 

 projection. A weak, or washed out appearance under the microscope 

 is sure to be even less satisfactory on the screen. 



(B) The thickness of the sections may vary from i/* to 40/4. But 

 one must remember that thick sections are adapted for low powers 

 only, while thin sections, if well stained, may be used with both high 

 and low objectives. 



The size of the object which one wishes to project determines the 

 objectives to be used. By consulting the table one can get a fair idea 

 of the size of object which each objective will satisfactorily project. 

 An excellent plan to follow is that for ordinary microscopic study (see 

 p. 102), that is, use first a low power to show the object as a whole, 

 then a higher one for details. . 



422. Projection of Living Objects. If living objects are to 

 be used with the projection microscope it is nececsary to eliminate 

 more of the heat rays than a single water bath can absorb. The speci- 

 men cooler will serve for a short demonstration, but if one wishes to 

 have the specimen under the instrument for 10 minutes or more it will 

 be necessary to introduce a second large water tank in front of the lamp 

 condenser and to use the specimen cooler also. For blood circulation 

 and living phenomena generally the microscope itself is much more 

 satisfactory than the projection microscope. 



423. A practical suggestion is made by L,ewis Wright in his book 

 on optical projection, and that is to warm the objective before using it 

 for showing the circulation of the blood or in any case when a moist 

 object is under it. If the objective is cold the vapor from the object 

 will be condensed on the objective and make satisfactory projection 

 impossible. 



424. Masks for Projection Preparations. The light used for 

 projection is so brilliant that it is practically impossible to arrange the 

 object under the objective with rapidity and certainty unless there is 

 some kind of guide. The best one found so far is a mask on the back 

 of the slide with an opening for the preparation to be shown. This 

 mask should be made of black paper. One can cut the holes in it with 

 scissors or with ticket punches. With the specimens properly masked, 

 and the parts of the apparatus lustreless black, the operator can work 

 with rapidity and also with comfort. (Fig. 211.) 



