78 



THE MICROSCOPE 



Bemoving 

 oil from 

 immersion 



Keeping 

 object 



Oorrections 

 of object 



become dirty, and in most cases the dirt will be on the front 

 surface. This should be cleaned with soft silk or very soft 

 chamois leather, but it must always be remembered that dust 

 consists in many cases of hard particles, often harder than glass, 

 and if these are rubbed upon the surface of the lenses they will 

 leave fine scratches. Hence the correct method is to wipe very 

 gently and so to remove the small particles and not to grind them 

 on to the surfaces. 



The lens which requires most cleaning is the front of an oil 

 immersion, which is necessarily continually covered with cedar- 

 wood oil. The oil should always be removed when the lens 

 is put away after use. Oil can be removed with xylol, 

 benzol, or spirits of wine, but care should be taken not to 

 use too much of this liquid, so that there is no danger of 

 its getting into the interior of the lenses. A piece of filter 

 paper or blotting paper moistened with xylol or benzol and 

 lightly wiped over the front surface wiU remove the oil with- 

 out rubbing. 



It is advisable to keep object glasses in the dust- tight metal 

 boxes in which they are suppUed when they are not in use. They 

 win be safe except in a very moist atmosphere in a dust-tight 

 nosepiece or in the boxes of the Sloan object glass changer. If 

 object glasses show dirt on the interior surfaces or any other 

 defects, they should be returned to the manufacturers, who alone 

 can satisfactorily put these matters right and see that the lenses 

 are in adjustment. High-power object glasses can be put out of 

 order by the slightest error in putting the component lenses 

 together. If a piece of dirt prevents one of the cells from screwing 

 quite home, it is sufficient to destroy its performance. An object 

 glass on the table when not in use should always be stood with 

 its front lens upwards to prevent dust from accumulating on its 

 back- surface. 



The reasons for constructing an object glass out of a number 

 of separate lenses in order to correct its aberrations wiU be 

 discussed in a more complete treatise referred to in the preface, 

 but one characteristic of a corrected lens should be thoroughly 

 grasped. Any lens or combination of lenses can be made to form 

 an image of an object at many difEerent positions. If a lens, such 

 as a bull's-eye, be put in front of a lamp, it can be moved to and 

 fro from the lamp till a position is found where it wiU form a 

 picture of the lamp on a wall ten feet away. If a card be now 

 interposed at a distance of only two feet from the lamp, a slight 

 movement of the lens away from the lamp will form the picture 

 upon the card instead of the wall. In the same way, if the length 

 of the drawtube of a microscope be altered, a slight movement of 

 the object glass wiU bring the image which it forms to the correct 

 position for the eyepiece to render it sharply defined. Any two 

 positions where an object and its image are situated are called 



