THE MECHANICAL STAGE. HOW TO FOCUS 177 



motion, as well as the separate rectangular ones, and gives great 

 facility for instantly producing any motion required without remov- 

 ing the hand from its position ; a most desirable attribute of a stage 

 when the rapid movements of a living and minute organism are 

 being followed. 



It still further enhances such a stage if a pinion is carried right 

 through the stage with a milled head at each end. 



A new stage devised by Watson has in it some features of interest, 

 a principal one being that the milled head controlling the horizontal 

 movements is in a fixed position ; in other words, does not travel 

 with the plate. This is shown in fig. 136. A is a ball upon which the 

 turning of the screw takes place ; it will be noticed that this ball has 

 a groove in it into which grease or dust can drift without affecting 

 the motion. The cap B covers the ball when fitted together. The 

 manner in which verniers are fitted is shown at D, D, and the screw 

 for adjusting the vertical rack movement of the stage is shown at C. 

 Fig. 137 shows the manner in which the plate E is attached to the 

 stationary screw; while fig. 138 indicates the careful manner in 

 which this stage is sprung to counteract continuous wear. The saw- 

 cuts shown are compressed by means of screws which are situated at 

 the points F F, G G, and any amount of wear can be corrected by the 

 use of these screws in these slots. 



The, aperture in the stage should always be larye, not less than 1^ 

 inches in diameter. There ought always to be space enough above 

 the ordinary slip when it is in position to permit of the easy inser- 

 tion of the index finger, for by its proper use, focussing with the 

 highest powers may be greatly facilitated. The object is to raise or 

 lower the slip, as the objective approaches the object, so as to dis- 

 cover how nearly it may be to contact with the front lens of a high 

 power in approaching focus. The focal distance should always be 

 felt, and not sought with the eye. 



Let it be supposed that we are using a dry object-glass with a full 

 aperture, and consequently short working distance. With the right 

 hand the coarse adjustment is worked ; with the elbow of the left 

 arm on the table, the second finger of the left hand resting on an 

 immovable part of the stage, which steadies the whole hand, the 

 index finger should rest lightly on the edge of the slip, and the 

 thumb be so placed as to graze the objective as it advances towards 

 the slip. The touch of the thumb indicates whether the objective 

 is an inch oft' or only a quarter of an inch away from the cover of 

 the slip. The movement of the coarse adjustment may be rapid up 

 to ^th or |-th of an inch, but after this there must be a cautious but 

 steady advance. The body may be racked down until by gentle 

 upward movement the slip is found to touch the front of the objective ; 

 then proceed cautiously by delicately lifting the slip from time to 

 time, by doing which we can proceed in perfect safety until the focus 

 of the object is obtained. In this way focussing becomes easy and 

 rapid, a matter of touch, and not of discontinuous procedure to 

 ' discover where the front of the lens is ' a search requiring a hand 

 glass and often, with its cumbrousness, considerable loss of time. 

 The above simple plan with brief practice will enable the operator to 



