132 THE MICROSCOPE. 



image shows clean outlines, then regulate the light with the 

 aid of the diaphragms placed under the optical system of the 

 apparatus. In every case, when artificial light is used, one 

 should place a lens between the lamp and the mirror, in order 

 to obtain parallel rays. 



If it is desired to resolve difficult tests with the aid of an im- 

 mersion objective, raise completely the condenser, putting its 

 surface in conjunction with that of the slide carrying the object 

 by means of a drop of oil or glycerine or of the immersion liquid 

 if one employs a homogeneous lens. This permits the use of 

 very oblique rays which, without this precaution, would have 

 suffered total reflection. It only remains now to diminish the 

 intensity of the light by means of the diaphragms, if they are 

 capable of being adjusted at different distances from the stage. 

 Remove them from the axis in order to have the oblique light 

 only on one side. If, with these arrangements, the object does 

 not appear as is desired, take off all the diaphragms so as to ex- 

 pose completely the condenser, incline the mirror to the right 

 or to the left, and when its position seems favorable, interpose 

 between the lens and the instrument (very near the mirror) two 

 thin metallic screens that can be brought near to each other 

 more and more, until the moment when the extremities of the 

 field begin to color. Then, almost always, at that moment, the 

 least movement of the mirror is sufficient to bring out the most 

 delicate details. 



By this method, we easily resolve with the white light of a 

 lamp, the Navicula affinis and even the difficult Amphipleura. 



If one desires, the lens can be changed for a balloon full of 

 ammoniacal sulphate of copper, which allows but one blue ray 

 of light to pass. This is particularly favorable for the resolution. 



However, in the majority of the cases, this disposition is use- 

 less for direct observation because of the important loss of light 

 and of the facility with which one can pass through the field all 

 the colors of the spectrum by simply bringing together the two 

 screens above alluded to. 



The study of certain colored bodies and particularly of the 

 bacilli, is conducted with white light while leaving the condenser 

 with its largest aperture and the mirror absolutely in the axis 

 of the microsc >pe. This is Koch's method. Le Micrographe 

 Preparateur, Jime, 1893. 



