16*4 THE MICROSCOPE. 



illuminating and polarising apnarntu*. or other auxiliaries 

 which are placed underneath the object. The tube and its 

 frame are affixed to a dovetailed sliding bar h, which can 

 be easily moved up or down, or taken off for conveniently 

 attaching the various apparatus. This sliding bar fits into 

 a second sliding bar, which, by means of a milled-head 

 screw, moving a rack and pinion, regulates the distance of 

 the apparatus from the stage. 



Directions for Use by Day or Lamplight. — In the adjust- 

 ment of the compound body of the microscope with the 

 illuminator above described, two important results are to 

 be sought — first, their centricity, and secondly, the fittest 

 condensation of the light to be employed. With regard to 

 the first, place the illuminator in the cylindrical tube, and 

 press upwards the sliding bar in its place, until checked 

 by the stop ; move the microscope body either vertically 

 or inclined for convenient use ; and with the rack and 

 pinion which regulates the sliding bar, bring the illu- 

 minating lens to a level with the upper surface of the 

 object stage ; then move the arm which holds the micro- 

 soope body to the right, until it meets the stop, whereby 

 its central position is attained ; adjust the reflecting mirror 

 so as to throw light up the illuminator, and place upon 

 the mirror a piece of clean white paper to obtain a uniform 

 disc of light. Then put on the low eye-piece, and a low 

 power (the half-inch), as more convenient for the mere 

 adjustment of the instrument ; place a transparent object 

 on the stage, adjust the microscope- tube, until vision is 

 obtained of the object ; then remove the object, and take 

 off the cap of the eye-piece, and in its place fix on the eye- 

 glass called the " centering eye-glass," described below, 

 which will be found greatly to facilitate the adjustment 

 now under consideration, namely, the centering of the 

 compound body of the microscope with the illuminating 

 apparatus of whatever description. 1 The centering-glass, 

 being thus affixed to the top of the eye-piece, is then to 



(1) 7 his centering-glass consists of a tubular cap containing two plano-convex 

 lenses, whi> h are applied and adjusted so that the image of the aperture in the 

 object-glass, and the images of the apertures at the lenses and in the diaphragms 

 contained in the tube which holds the illuminating combination, may all be in 

 focus at the same time, as with the same adjustment they may be brought suffi- 

 ciently near in focus to recognise their ecutricity. 



