260 PROJECTION MICROSCOPE [_CH. IX a 



when it is to be used the ordinary objective support and focus ar- 

 rangements are removed bodily and this apparatus put in place (Fig. 

 204, B). It is focused by sliding it along the bed-plate. 



By having an ordinary lantern slide carrier in position next the 

 condenser one may use this apparatus also for lantern slide projection 

 by removing both the microscope and the stage and putting in place a 

 block carrying the ordinary projection objective. A double lantern is, 

 however, more satisfactory as no time is lost in making the change of 

 apparatus, and both lanterns can be in perfect adjustment for the special 

 purpose in hand. 



411. Water Baths for Removing Excessive Heat. For 

 removing the excessive heat of the light beam tanks of water are 

 placed in its path. In the apparatus shown in Fig. 204 the main tank 

 is between the condenser lenses (see also Fig. 209 (5) and Fig. 213 A). 

 In Fig. 205 D the water bath has been removed. In addition to this 

 large water bath a special one called a specimen cooler is placed imme- 

 diately under the stage (Fig. 204 C, D, Fig. 208 G). For lower 

 powers where the specimen is near the condenser and hence in a wide 

 beam the large water bath usually suffices. (See Fig. 209, 2, 3.) If 

 the object is in or near the focus (Fig. 209 (i)), the heat is concen- 

 trated as well as the light and the specimen cooler is necessary to 

 protect the specimen: 



In some projection microscopes the cooling tank is wholly free from 

 the condenser. That is an advantage for the contact of the tank with 

 the metal of the condenser serves to conduct much heat to the water 

 and it becomes hot not from the heat absorbed from the light, but 

 from the heated metal of the condenser mounting. To overcome this 

 difficulty one may have two cooling cells and when one is heated a 

 cold one can be put in its place. Sometimes the water is kept cold by 

 a constant change of the water. This is liable to give rise to currents 

 and consequently a shimmering or wavering of the light on the screen. 



412. Liquid for Removing Heat. In the older works a 

 solution of alum was always advised as the heat absorber. It has 

 been found by careful quantitative experiments that distilled water is 

 quite as efficient. It is much cleaner and more transparent than alum 

 solutions. See Physical Review, vol. i, p. 14. 



413. Objectives to Use in Micro-Projection. Without 

 oculars the lowest power ordinarly used with the microscope is about 

 75 mm. focus. From this lowest point one will find objectives of 64, 



