270 EXHIBITION WITH PROJECTION MICROSCOPE [Ch. IX Conduct of an Exhibition or Demonstration with the Projection Microscope § 400. What is said in Ch. I, § 21-40 is entirely applicable to the projection microscope by substituting microscopic specimens for the lantern slides. Only from the greater difficulty and pre¬ cision demanded in using the projection microscope, it is impera¬ tive that the operator be prepared, hence the greater necessity of making certain that everything is in absolute order before the lecture begins. If any of the projection objectives (i. e., those of 125 to 20 mm. focus) have iris diaphragms, open these as widely as possible. Never try to project with the iris of the objective partly closed. Fig. 148. Slide-Tray with Masked Preparations to be Used in Projection. (About X size). Three series arc here represented on different sized slides. The sections to be shown arc not covered with the masking paper. The numeral on the side give the number of the series (ser. 90, ser. 17, ser. 15). On each slide is also the number of the slide in the series as ser. 15, slide 57, 63, 67, etc.