To obtain the best results, time and care must be given to the illumination of the object. The lamp and microscope having been placed in position, a low power is first used and the smallest diaphragm. On looking through the microscope it will probably be observed that the image of the diaphragm is not in the centre of the field; by moving the centring screw of the condenser this may be adjusted. The low power is then replaced by a high power, the largest diaphragm used, and the bacteria or diatom brought into focus. The diaphragm must now be replaced by one of medium size, and by racking the condenser up and down, a point will be arrived at when the image of the edge of the flame appears as an intensely bright band of light. If this is not exactly in the centre of the field the centring screw of the condenser must again be adjusted. With regard to the use of diaphragms, various sizes should be tried while focussing with the fine adjustment, at the same time using the correction colour; in this way we obtain the sharpest possible image. When the condenser has been accurately centred, it will still be necessary to focus it for each individual specimen, so as to correct for difference in the thickness of slides and the layers of mounting medium. Correction for different thickness of cover-glasses must be made by the aid of the collar adjustment in the following way: a high-power eye-piece is substituted for the ordinary eye-piece, and the faults in the image will thereby be intensified. By moving the collar completely round, first in one direction and then in the other, while carefully observing the effect of the image, it will be seen to become obviously worse whichever way the collar is turned. The collar must then be turned through gradually diminishing distances until an intermediate point is reached at which the best image results with the high-power eye-piece, and on replacing this by the low-power eye-piece the sharpest possible image will be obtained.