THE MICROSCOPE. 131 



utilize by preference the light of white clouds or that reflected 

 from a wall. Blue sky does not permit any difficult observations. 

 The sun should never be employed. In case it shines on the 

 window, hang over it a thin white curtain which should produce 

 an excellent light. 



If artificial light is employed, use a lamp giving a white and 

 absolutely fixed flame and one presenting the smallest surface 

 possible. This condition is the only one which permits the most 

 delicate observations. The light of incandescent lamps and of 

 ordinary gas gives also some very good results but their use is 

 less convenient and more tiring. 



Illuminating the Object. — In this we have two cases to 

 consider: (1) When the microscopist owns an instrument which 

 is furnished with a condenser, and (2) when he has no condenser. 

 We begin with the second case. 



The microscope being placed at about 20 centimetres (8 inch- 

 es) from the flame and so that the latter may be nearly on a level 

 with the stage. Direct by means of the mirror the luminous 

 rays on the preparation ; use the plain mirror with very low 

 power objectives, and the concave with ones of high-power. 

 Regulate the quantity of light by means of the diaphragms. 

 The higher the power of the objective the smaller the diaphragm 

 ought to be and the nearer to the preparation. A too brightly 

 lighted field is not only dangerous to the eyes, but even prevents 

 seeing well, for the objects will be drowned in light. 



If the objects that we examine present some fine details, pearls, 

 stride, etc., like Diatomacea?, move away the diaphragms, or use 

 the largest if it is a revolving diaphragm ; then incline the mir- 

 ror to the right or to the left in order to obtain some oblique 

 rays which project the image of finest details, exaggerating their 

 importance and permitting us to perceive them better. The de- 

 sired result is to see well with the least possible oblique light, 

 for a too oblique light gives a badly illuminated field and brings 

 about a distortion of images. 



If the microscopist owns a condenser, Abbe's or any other 

 with large aperture, he will not only have much more facility 

 for ordinary study, but he will also be able to make some obser- 

 vations which would be impossible without this instrument. 



For ordinary work with low-powers, one should employ the 

 concave mirror, regulate the height of the condenser until the 



