Ch. VI] THE PROJECTION MICROSCOPE 201 



excellent masking paper can be prepared by soaking white paper in a saturated 

 aqueous solution of Orange G for 10 to 15 minutes, and then hanging it up until 

 dry. The paper is then cut into pieces the size of the cover-glass. Holes are made 

 opposite the sections to be demonstrated (fig. 120), and the paper pasted to the 

 cover-glass. It is put on the cover-glass and not on the under side of the slide be- 

 cause if on the slide it would prevent the conduction of the heat absorbed. If the 

 slide rests against the metal or glass stage the absorbed heat is largely conducted 

 away by the stage water cell or the metal stage. 



If one wishes to remove the mask from the cover the safest way is to place a 

 piece of wet blotting paper on the mask to soften it. It can then be easily re- 

 moved and the cover-glass cleaned with a moist cloth. 



§ 313. Objectives, oculars, and amplifiers to use in projection. — 



Objectives of the photographic type from 100 to 16 mm. equivalent 

 focus are imexcelled. They should not be used with oculars. Of 

 ordinary objectives, all powers can be used, but for demonstrations 

 before large classes a 4 mm. is the highest power found reaUy satis- 

 factory. The ones most used are of 16, 10, 8 mm. focus. Oculars 

 of the projection or the ordinary type answer well. If the projection 

 type is used one must rotate the eye-lens until there is a sharp image 

 of the diaphragm on the screen to get the best image of the object. 



If one is to use an amplifier with the objective, but no ocular, the 

 iacrease in size should not be over 1.5 to 2.5 beyond that given by 

 the objective. For example if the screen image with the objective 

 alone were 500 diameters, the amplifier should not enlarge it beyond 

 750 or 1000. 



The ones used by the author are for the large tube of the micro- 

 scope (fig. 112) and are nearly 5 cm. in diameter to fit the microscope 

 tube. Their virtual foci are 20 and 10 cm. (5 and 10 diopter con- 

 caves). 



§ 314. Centering the optical parts on one axis. — This is one of 

 the most important procedures of all and no good projection can be 

 accompKshed without it. The easiest way is to first arrange the 

 crater of the arc lamp, the central point of the large condenser, and 

 the microscope objective all at the same height from the base board 

 (fig. in). If then the lamp is turned on and the objective placed 

 in the focus of the main condenser cone, the image of the crater of 

 the arc lamp should be formed on the end of the objective, the brightest 

 part on the front lens. If the image is to one side, above or below, 

 then the microscope should be raised, or lowered. After being once 



