MICROSCOPY 1 1 



is experienced in correcting such faults, it is best to return the instrument to 

 the maker for repairs. 



Important as proper illumination and focusing are, satisfactory observations 

 cannot be made if the lenses be dirty, therefore, they are carefully protected 

 from dust and moisture. Keeping an ocular in the instrument when not in 

 use, prevents dust from entering the tube and objective. When the field of 

 vision is obscured by specks, they are usually upon the ocular. To determine 

 this, rotate the ocular while looking through it. If the dust is on the ocular, the 

 specks move when the ocular does. Wipe off the most exposed surface first, if 

 that does not correct the fault, carefully clean all the surfaces of the ocular lenses, 

 one after the other until the ocular is perfectly clear. Should an objective be 

 dirty, first clean the outside, if there is dust inside an ocular, try to remove it 

 with a fine camel's hair brush. Never take an ocular apart; if that seems neces- 

 sary, let the maker do it. Only fine, soft, clean linen or silk or Japanese lens 

 paper should be used to wipe lenses. They should never be rubbed hard. 

 If a solvent is required to clean lenses, wet the lens paper with xylol or chloro- 

 form, nothing else, and rapidly dry the lens after wiping. Immersion oil should 

 never be left on a lens when not in use; it should be wiped off. The substage 

 condenser and mirror require the same careful attention and must be as clean 

 as oculars and objectives. Should a lens be scratched or its finish etched or 

 marred, the services of a lens grinder will be required to restore it. 



THE DARK-FIELD MICROSCOPE 



Unstained microorganisms are most distinctly visible and their motility 

 and morphology can be most accurately observed when they are situated in a 

 medium that has a dark background and are illuminated by rays of light that 

 pass obliquely through the fluid between slide and cover glass without entering 

 the objective. In fact, organisms such as the treponema pallidum, which can- 

 not be satisfactorily observed in ordinary hanging-drop preparations, are 

 distinctly visible and clearly outlined in dark-field preparations, notwithstanding 

 the fact that magnification is the same in each case. 



For this desirable method of observation one requires a microscope fitted 

 with a dark-field condenser. Such a condenser can be affixed to any ordinary 

 microscope but on account of the time necessary to properly adjust the micro- 

 scope for this work, the only practical method for routine examinations, at 

 present, is to equip a microscope with a dark field, adjust it, and fix it in position 

 and only use it for this method of observation. 



The one great value of this apparatus, at present, in routine diagnostic work, 

 is in the examination of fluids and scrapings from suspicious lesions to detect 

 the presence of the treponema pallidum. 



More intense illumination is required for the dark-field microscope than for 

 the ordinary and can be obtained most satisfactorily from one of the numerous 

 electric lamps made especially for the purpose. A lens or round flask filled 

 with water must be so placed between the source of light and the microscope, 

 that by manipulating the mirror of the microscope it is possible to get the whole 



