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CH. IX] PROJECTION WITH SUBSTAGE CONDENSER 235
main condenser (fig. 132 1^) have a focus of 150 to 200 mm. (6 to 8 inches). With such a main condenser one can do successful projection 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 entering 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.