THE MICROSCOPE. 29 



power the mirror may be placed at such a distance from 

 the object as is necessary to illuminate the entire field, while 

 with a high power the rays should be brought to a focus in 

 or near to the object. The light is exactly focalised en 

 the object, when a window bar, that may be brought into 

 the field, can be seen sharp and distinct, or when the shape 

 of a lamp flame can be distinctly seen. With artificial 

 light it is often inconvenient to have the flame thus exactly 

 focalised on the object, and therefore the lamp should be 

 drawn a little nearer to the mirror to avoid this. The 

 mirror should in general be so placed that the cone of rays 

 is reflected from it at a right angle. One generally arranges 

 the mirror so as to send the light directly through the object, 

 but sometimes for the resolution of fine structure it is im- 

 portant to throw the light into the object obliquely. By 

 such means one side of the object is thrown into shadow. 

 In the more expensive microscopes the mirror has two 

 surfaces, one concave, the other flat. The flat surface 

 is used when the light is to be focalised on the object 

 with an achromatic condenser ( 52). 



44. The Diaphragm usually consists of a blackened 

 brass disc (M, Fig. i) placed below the stage. It has 

 apertures of various sizes, which can be successively brought 

 under the aperture in the stage (L, Fig. i). It is very 

 important to attend carefully to the use of the diaphragm, 

 and so regulate the amount of light transmitted through 

 the object ; for if a glare of light be allowed to pass through 

 the object, the finer shadows are lost, and definition is thus 

 rendered indistinct. With a high power a small aperture is 

 employed. The diaphragm is, for convenience, usually 

 placed about half-an-inch below the object. For lenses of 

 very short focus, however, T ^ inch and upwards, it is very 

 important to have the diaphragm close to the slide. In 

 Hartnack's more expensive microscopes this is effected by 

 a movable tube having one end partially closed by a dia- 

 phragm. Every objective has its appropriate diaphragm, 

 the aperture being smallest for the lenses of shortest focus. 

 The advantage of this arrangement is that the diaphragm 

 can be pushed close up to the slide as it lies on the stage, 



