256 THE USE OF THE MICROSCOI'E 



with the full aperture of the objective, and a condenser cone 

 as large as possible without glare (perhaps four-fifths of the 

 objective aperture), and note distinctness of the trans- 

 verse lines, and faint visibility of the (network or) dots. 

 Cut down the aperture of the objective by its iris to about 

 0.8 or 0,7, reducing the condenser cone correspondingly 

 (so as to avoid glare). Note the lessened visibility of the 

 transverse lines. Focus an oil-immersion objective 90, 

 of 1.3 aperture on the object, using a ^^ condenser cone, 

 and observe the (network or) dots. Reduce the condenser 

 cone gradually, and note that the (network and) dots 

 ultimately vanish. (Spitta.) 



21. Centering the Condenser by the Condenser Light 

 Circle on the Back of the Objective. — Fit tightly a round 

 cardboard cap (like a pill-box lid) with a central perforation, 

 on the top of the microscope tube, instead of an eyepiece. 

 Using an oil-immersion objective focused on a well-stained 

 object in immersion oil, with the condenser immersed by 

 water under a slide 1.0 millimeter thick, notice whether 

 the edges of the iris of the condenser (as seen on the back 

 of the objective) are concentric, when opened sufficiently, 

 with the edges of the back lens of the objective. If not, 

 alter the centering of the condenser until the centers coin- 

 cide. (If there is no centering sleeve to the condenser, 

 but a sleeve that can be tightened by a screw, a piece of 

 paper, of the right thickness, gummed on the right place on 

 the side of the condenser, will usually set matters right.) 

 (Coles.) 



22. Aperture of the Condenser Cone. — With the micro- 

 scope as in 21, but having the eyepiece in place, center a 

 corrected lens magnifying about 15 times (the lens part of a 

 15-times compensating eyepiece will suit) over the eyepoint, 

 and observe the condenser light circle in the exit pupil. The 

 aperture of the condenser cone may be estimated by the 

 ratio of its diameter at the eyepoint to that of the outer 

 dim circle, which is the image of the objective aperture 

 circle. (This ratio may be measured directly on the back 

 of the objective, by screwing a low objective into the 



