THE CARE OF THE MICROSCOPE 117 



region of most distinct vision, the fovea, the cross section of a cone is a cir- 

 cular area of diameter '002 mm. Now if not more than one cone is stimulated 

 the resulting impression will be that of a single point of light. 1 Therefore 

 errors will be absolutely negligible if they give rise to such small confusion 

 circles that they do not extend over more than one foveal cone. 



SECTION III. THE CARE OF THE MICROSCOPE. 



The microscope must be kept at as uniform a temperature as possible and 

 away from direct sunlight and all other sources of heat, because the lenses 

 are held in position by Canada balsam, and they would be displaced and the 

 instrument put out of order if the balsam were to be melted. It is also 

 essential to protect the microscope from dust, which may best be done by 

 standing it on a piece of thick felt or india-rubber on the bench, and cover- 

 ing it when not in use with a glass shade. 



Objectives and eyepieces should always be wiped with a piece of soft linen 

 before use, to ensure their being absolutely clean. If on looking down the 

 microscope a speck of dust be seen in the field, one must find out where it 

 is in order to wipe it off. To determine the position of the speck, first rotate 

 the eyepiece ; if the dust be on one or other of these lenses it will of course 

 alter its position ; and if rotation of the eyepiece do not alter its position, 

 then it is on the objective. By holding the lenses up to the light some 

 distance from the eye, it can be seen if they are cloudy or if specks of dust 

 adhere to them. 



To clean the front lens of the objective, rub it with an absolutely clean 

 piece of fine linen ; if this fail to clean it, take a piece of elder pith, strip off 

 a thin layer, and with the clean surface so exposed gently rub the lens. 



If cedar-wood oil, Canada balsam, or dammar varnish be sticking to the 

 lens, moisten the cloth with a drop of xylol, and gently wipe the lens. An 

 excess of xylol must not be used nor should xylol be poured on to the objec- 

 tive, for fear that it should penetrate between the lenses and their mountings 

 and dissolve the balsam holding them in position. 



When it is necessary to examine preparations in caustic potash, acids or 

 other chemical reagent, great care must be taken to keep the lenses from 

 coming in contact with the reagent : but if by accident the lens should be 

 soiled, wash it at once in distilled water and dry with a soft linen rag. 



If the objective be cloudy, and cleaning the outer lens does not remove 

 the cloudiness, it must not be unscrewed to clean the inner lens, but should 

 be sent to the maker, who is the only person capable of putting it right. 



Objectives should be carefully protected against the slightest shocks 

 or falls. 



The eyepiece and the Abbe condenser can be cleaned in the same way as 

 the objective, but these lenses are much more accessible and infinitely less 

 delicate. The mirror can also be cleaned in the same way. 



Before putting the microscope away, always wipe the eyepiece and objec- 

 tives, and remove every trace of oil from the immersion lens. 



The stand should be wiped frequently with a chamois leather, and rubbed 

 in the direction in which the lacquer has been applied. Should the stand 

 be accidentally soiled with balsam or cedar-wood oil, apply a little xylol on 

 a soft cloth, and remove it at once with a chamois leather ; if too much xylol 

 be used or if it be not carefully wiped off it will dissolve the lacquer from 

 the metal. 



1 For simplicity of explanation the question of diffraction is ignored in this case. 



