MICROSCOPIC ACCESSORIES. 



39 



which is the ten-millionth part of the quadrant of the 

 meridian of Paris. The multiples are made by prefixing 

 Greek names of numbers, and the subdivisions by prefix- 

 ing Latin names. Thus, for decimal multiples, we have 

 deco, hecto, kilo, and myrio ; and, for decimal subdivisions, 

 deci, centi, and milli. The following may serve for con- 

 verting subdivisions of the metre into English equiva- 

 lents : 



A millimetre equals 0.03937 English inches. 



A centimetre " 0.39371 " 



A decimetre " 3.93708 " 



One inch = 2.539954 centimetres, or 25.39954 millimetres. 



'For drawing microscopic objects the camera lucida will 

 be found useful. This is a small glass prism attached to 

 the eye-piece. The microscope is inclined horizontally, 



FIG. 17. 



Oberhauser's Drawing Apparatus. 



and the observer, looking into the prism, sees the object 

 directly under his eye, so that its outlines may be drawn 

 on a piece of paper placed on the table. Some practice, 

 however, is needed for satisfactory results. For the up- 

 right stands of German and French microscopes, the camera 

 lucida of Chevalier & Oberhauser is available. This is a 

 prism in a rectangular tube, in front of which is the eye- 

 piece, carrying a small glass prism (c, Fig. 17), surrounded 



