CH. IX] PROJECTION WITH SUBSTAGE CONDENSER 235 



main condenser (fig. 132 La) have a focus of 150 to 200 mm. (6 to 8 

 inches). With such a main condenser one can do successful pro- 

 jection with objectives from 125 to 4 mm. focus. The aperture 

 will not be completely filled in the 8, 6 and 4 mm. objectives, but 

 brilliant screen images are obtained even with them for a 7 . 5 meter 

 (25 ft.) screen and 12 amperes of direct current. One can also use 

 a -5 diopter amplifier when good specimens are projected. (For 

 the position of the objective and specimen see 376). 



With a substage condenser there is a great loss of light from 

 reflection and absorption so that the increased aperture hardly 

 compensates for it, and the increased detail is lost for the observers 

 are too far from the screen to see them (see 35ga). 



For special demonstrations and for drawing where the observers 

 are very close to the screen, the substage condenser and also an 

 ocular are advantageous, and for fine details, necessary (see 401, 

 477). 



SUITABLE ROOM AND SCREEN FOR MICRO-PROJECTION 



360. From the small size of the objective for micro-projection 

 the image on the screen cannot be made as bright as with the magic 

 lantern, hence it is necessary in micro-projection to have a room 

 that can be made very dark ; and the devices for cutting out stray 

 light, bellows, objective hood and shield must be efficient (fig. 

 133. 139)- 



359a. i. Wright, p. 212, says: "The iris of the substage condenser is 

 opened or closed until the best effect is produced." This can mean only that 

 not the whole cone of light is used in some cases. 



2. To determine the amount of aperture of the objective used in projection, 

 take a thick piece of smoked mica or combine brown and blue, or deep red and 

 blue, or red and green glass and put them over the front of the objective to 

 soften the light. Or one might hold one of these light softeners just in front 

 of the eye. Then in any given case look along the microscope tube directly 

 toward the light, and the aperture of the objective actually filled by the enter- 

 ing cone of light can be seen. If the entire aperture is used, the back lens of 

 the objective will be filled with light; if only a part of the aperture, then there 

 will be a central brilliant circle and a dark zone of glass surrounding it (fig. 151). 



It must be remembered too that the large specimen cooler (fig. 121, 134) 

 cannot be used with a substage condenser; and in our opinion this overbalances 

 any advantage that the substage condenser might yield for demonstrations to 

 large classes. 



