_Directions for using Gillett’s Condenser._--In the adjustment of the compound body of the microscope for using with Gillett’s illuminator, one or two important points should be observed--first, centricity; and secondly, the fittest compensation of the light to be employed. With regard to the first, place the illuminator in the cylindrical tube, and press upwards the sliding bar _k_ 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 illuminating lens to a level with the upper surface of the object-stage; then move the arm which holds the microscope 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 “centring eye-glass,”[26] which will be found greatly to facilitate the adjustment now under consideration, namely, the centring of the compound body of the microscope with the illuminating apparatus of whatever description. The centring-glass, being thus affixed to the top of the eye-piece, is adjusted by its sliding-tube (without disturbing the microscope-tube) until the images of the diaphragms in the object-glass and centring lens are distinctly seen. The illuminator should now be moved by means of the left-hand screw on the secondary stage while looking through the microscope, to enable the observer to recognize the diaphragm belonging to the illuminator, and by means of the two adjusting screws to place this diaphragm central with the others: thus the first condition, that of centricity, will be accomplished. Remove the white paper from the mirror, and also the centring-glass, and replace the cap on the eye-piece, also the object on the stage, of which distinct vision should then be obtained by the rack and pinion, or fine screw adjustment, should it have become deranged. [Illustration: Fig. 127.--The Ross Improved Achromatic Condenser, with diaphragm stops.]