THE ANGULAR MICROMETER 
applicable for use 
with microscope 
slides and is useful 
for very slight errors 
of focus. Figure 4 
illustrates this meth- 
od. If a beam of 
light from a radiant 
point 0 on a plane 
mirror is directed 
through the nar- 
rowed orifice of an 
iris diaphragm and 
strikes the condenser 
at one side of the 
center, the light will 
pass through the 
microscope along the 
path 0-A-F. If di- 
rected through the 
opposite side of the 
condenser, by mov- 
ing the diaphragm, 
the beam wil 
traverse the path 
O'-B-F'. It is evi- 
dent that if an ob- 
ject is placed on the 
stage in such a posi- 
tion that the point- 
er-hair in the ocular 
apparently touches 
its e d ge, a move- 
ment of the dia- 
phragm will be fol- 
lowed by a move- 
ment of the image 
past the pointer-tip 
if the object is out 
Fig. 4. Method for detecting focal errors. By means 
of the iris-diaphragm a beam of light is made to pass over 
either path 0-A-F or O'-B-F'. If the image in the micro- 
scope remains stationary the object is in focus. 
