102 
ELEMENTARY CHEMICAL MICROSCOPY 
is in focus, there being an aperture in the base in line with the 
optic axis or the base is provided with widely divergent legs. 
Figs. 46, 47 and 48 illustrate typical instruments of this class. 
In the Stead instrument, Fig. 46, the body tube is supported 
upon three adjustable legs. Focusing is done by hand by rais¬ 
ing or lowering the tube in a sleeve. When in focus the instru¬ 
ment is held in place by a clamping screw C. A vertical illumi- 
nator of the disk type forms an integral part of the instrument.^ 
The radiant in this case consists of a tiny incandescent electric 
lamp enclosed in a sleeve at right angles to the illuminator mount¬ 
ing. As the instrument is intended for low magnifications only, 
no fine adjustment is provided. 
A somewhat similar idea in illum¬ 
inator construction is found in the 
Tassin metallurgical microscope.^ 
In this instrument. Fig. 47, we find 
the illuminator of the form already 
Fig. 48. Leitz Metallurgical Microscope. 
described on page 86, Fig. 37, the radiant being either an 
electric or an acetylene lamp. The microscope itself has no 
^ See Stead, Work Shop Microscopes. J. Roy. Micro. Soc. 1909, 20, 22. 
^ For its application see Tassin, The Microstructure of Steel Castings, J. Ind. 
Eng. Chem., 6 (1913), 713. Metallography as Applied to Inspection, J. Ind. 
Eng. Chem., 6 (1914), 95. 
