ADJUSTMENT OF MICROSCOPE 13 



insert first one eye-piece, then the other : rotate each eye-piece, 

 and if any specks be seen to rotate with it they are on the 

 lenses of the eye-piece, -and must be removed with a soft 

 chamois leather, or a fine linen or silk cloth (old pocket- 

 handkerchief). Carefully examine the front lens of eachobjective, 

 and if any dirt be seen wipe the lens gently with a chamois 

 leather, or fine linen or silk cloth. Glycerine is apt to gain 

 access to the objective in careless hands ; when this is the case 

 the lens is to be washed with a jet of distilled water, and care- 

 fully dried. Since the lenses are often fixed with balsam, 

 great care must be taken that they should not be smeared 

 with Canada balsam, or Dammar : when this has happened the 

 lens should be gently rubbed with a cloth wetted with a very 

 little benzol, or alcohol. In all cases the cleaning of lenses 

 should be carried out as gently as possible, to avoid destroying 

 their polish. 



The best light for microscopic work is that reflected from 

 white clouds in a northern sky, and a window with a northern 

 aspect should be selected. Never use direct sunlight, and avoid 

 using artificial light. If the only available room has a south 

 aspect, a white blind is to be used, so as to cut off direct sun- 

 light, or a piece of white card may be fitted to the surface of 

 the mirror, so as to act as a less perfect reflector. 



The body of the microscope should be vertical ; with the short 

 microscopes now in use, the oblique position is quite un- 

 necessary, and very inconvenient when mounting in fluid media, 

 or irrigating with fluid reagents. 



Always examine an object with a low power first, and after- 

 wards, if necessary, with a higher power. It is a general principle 

 of microscopic practice that observations should be made with 

 the lowest possible power sufficient for distinct vision. Never 

 use the high power unless the object be covered with a cover- 

 slip. 



When a low power is used a larger hole of 'the diaphragm 

 below the stage is to be placed opposite the aperture in the 

 stage ; when a high power is used a smaller hole of the diaphragm 

 is necessary, otherwise the definition will not be satisfactory. 



Some difficulty will be felt at first \K finding the focus. There 



