264 PROJECTION MICROSCOPE \CH. IX 



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. 



(C) For minute objects like white blood corpuscles, etc., it is 

 necessary to use a high power and to have a small audience which can 

 be close to the screen, or a somewhat larger audience can see well by 

 using opera glasses. 



(D) For the circulation of the blood it is necessar}- to eliminate 

 the heat rays almost entirely. Nothing has proven so good as the 

 second or specimen cooler (§413). The conditions are still more 

 favorable if a circulation of cold water is established in the large water 

 bath also. This is easily done by the use of two large bottles. The 

 cold water can be siphoned or aspirated from an upper one and the 

 warm water allowed to flow off into a lower one. For this it is of much 

 advantage to have a tube in the bottom of the water bath in which to 

 introduce the cold water. The warm water will then flow off through 

 a tube in the top. One must remember that perfectly clean water must 

 be used for the water bath especially when a circulation is established, 

 for opaque particles in the water bath give undesirable shadows in the 

 image. 



(B) A practical suggestion is made by Lewis 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 satisf actor)' projection 

 impossible. 



§ 425. Masks for Projection Preparations. — The light used for 

 projection is so brilliant that it is practicall} 7 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 L 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 lusterless black, as suggested by Dr. 



