THE MICROSCOPE AND ITS ACCESSORIES. 



21 



metre or micro-millimetre, expressed shortly as a micron, or by 

 the Greek letter //,. Thus, each space of the ocular micrometer is 

 equal to 3-3 //,. 



With these data it is easy to estimate the size of any object. The 

 object is placed on the stage and focussed, always, of course, with 

 the same combination of lenses and ocular. If other lenses be used, 

 the value of the ocular divisions must be determined for this particular 

 combination. Once determined, they can be noted for future use. 



18. Artificial Illumination. White daylight is, of course, to be 

 preferred, but it is not always available. When artificial light has 

 to be used, a gas flame, e.g., an Argand burner, or other artificial 

 light, may be used. An ordinary paraffin lamp with a flat wick and 

 arranged to burn steadily does very well. An unsteady flame is 



FIG. 22. Micrometer Eye-piece. 

 (EG.) eye-glass ; (FG.) field- 

 glass ; (S.) scale. 



. 23. Microscope Lamp. 



very injurious to the eyes. It requires some care to find the exact 

 distance at which the lamp should be placed from the mirror. If 

 very intense light be required to examine a small part of a pre- 

 paration, turn the wick edge on to the mirror. Between it and the 

 mirror place a screen of white paper. If the direct rays from a 

 lamp are used, to correct the yellow rays place a sheet of pale blue 

 glass, or a glass globe containing a weak solution of ammonio- 

 sulphate of copper, between the light and the mirror, or a thin blue 

 glass may be placed on the stage under the preparation. A paraffin- 

 oil lamp may be used. Fig. 23 shows a cheap paraffin lamp sold by 

 Swift & Sons. A remarkably good light is obtained from the 



