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Transactions of the Society . 
The first error can be corrected by tightening one or both of 
the guides ; the second, by loosening them. The guides, therefore, 
must be neither too tight nor too loose. The first error is the more 
serious of the two. It is due to an unequal tension in the springs 
which throws the tip of the moving screw to a different spot on the 
bar against which it abuts. If this be not corrected, the screw will 
in time wear a depression in the brass bar on a different part of 
the bar against which it works, thus perpetuating the error. The 
second error is due to the guides being too tight, so that they bind 
and prevent the bars from making a true return. If not corrected, 
this error will be gradually eliminated with the wear of the 
frictional surfaces. 
4. The Substage Condenser and the Method of Making 
Barber’s Moist Chamber and Glass Needles. 
For critical illumination, the height of the moist chamber must 
be equal to the working focal distance of the substage condenser. 
The Abbe condenser can be used by removing the top lens. The 
focal distance of the remaining lens is almost one inch. In the 
Bausch and Lomb microscope the substage can easily be arranged 
to raise this lens sufficiently to have at least half its focal distance 
above the surface of the stage. This is ample, for one seldom 
requires a moist chamber as high as \ in. The focal distance of 
this lens can be reduced and its illuminating power correspondingly 
increased by placing the lens of a 10 X dissecting lens on top of it. 
This combination has a focal distance of about | in., and, if the 
substage can be raised to bring the top lens flush with the upper 
surface of the stage, all of this distance may be used for the height 
of the moist chamber. Better results are secured with a triple 
lens condenser with its top lens removed. Such a condenser from 
Leitz which I am using has a working focal distance of | in. One 
may also use condensers which are made with a specially long 
working distance for projection apparatus in which a cooling 
trough is placed between the condenser and the slide. 
If the working focal distance of the condenser be less than | in. 
it is well to have two moist chambers, one for critical work and 
the other, from § in. to J in. high, for ordinary work. This is 
advisable because it is easier to make needles for the higher 
chamber. 
The moist chamber is made of glass. The dimensions of a 
convenient size are given in fig. 4. The base is a fairly thin glass 
slide about 2f in. by 2 in. in size. The sides consist of strips of 
plate glass about in. long and J in. wide, and of a height 
determined upon by the available condenser. One end of the 
chamber is closed with a strip of glass of the same height as the 
sides, and backed by another strip a fraction higher in order to 
