194 METHODS OF MICROSCOPICAL RESEARCH. 



inevitably set up in the apparatus, and the plate, if 

 exposed during this vibration, would not be impressed 

 with a sharp image. Before drawing the shutter a 

 slip of card, covered on both sides with velvet, and 

 attached to a separate stand, should have been 

 interposed in the beam of light between object and 

 object glass, if the latter is of 1^ inches or lower 

 power; behind the stage if of higher power. With 

 this stop in position the shutter may be drawn 

 without exposing the plate. A few seconds are 

 then allowed for vibrations to subside ; the stop is 

 taken in the hand and held for a second touching 

 nothing, and then quickly removed and the exposure 

 commenced, the time being noted. If the exposure 

 is only to be of a few seconds' duration, the operator 

 still retains the stop in his hand and remains per- 

 fectly still while he counts the seconds on a watch, 

 or the vibrations of a ball attached to a string 39 

 inches long, and held in his other hand. The 

 instant the time expires the stop is replaced, the 

 shutter closed, and the slide withdrawn and carried 

 to the dark room; 2oz. of A solution are then poured 

 into the beaker (there is no need to be particular to 

 half a drachm or so, and it is easy to guess when 

 sufficient has been poured into the beaker). The 

 light is then turned very low, and the operator, 

 turning his back to the lamp, removes the plate, 

 passes the brush over it once more, and places it 

 face upwards in the developing dish ; then the 

 "pyro" solution from the beaker is poured on to the 

 plate with a steady and rapid motion, so that the 

 whole plate is flooded at once. The tray is then 

 put aside in a shady place for a few minutes, while 

 the operator measures out 25 drops (minims) of B. 



