74 THE MICROSCOPE. 
taken from different packages, you will find quite a proportion show 
an appearance resembling minute beads of moisture on the surface. 
This is due to a roughness of the glass, which prevents the light 
from passing through properly. If such glass is used on balsam 
mounts the defect is not noticeable, but if used for dry mounts an 
amount of sweating that would ordinarily be of little moment will so 
intensify the effect that the mount is declared ruined and the cell 
gets the blame. My experience with wax cells has been very great, 
and when carefully prepared I do not think they are any more liable 
to sweat than any form of cell that is hermetically sealed. If it were 
possible I would put up everything in a solid medium, such as bal- 
sam or Farrant’s solution. Such preparations will remain long after 
those dry or in fluid have been thrown away. 
272 Turrry-First Street, Carcaco. 
MICROMETER MEASUREMENTS. 
M. D. EWELL. 
N MY last communication I advocated the use of metal microme- 
ters uncovered, stating, however, that if necessary a temporary 
cover could be used. In the same connection I ought to have stated 
that if such a temporary cover should be used, it should always be 
used under precisely the same conditions, and the observer should 
be quite sure that both faces of such cover are parallel, otherwise 
the influence of refraction, the cover acting as a prism at some part 
of its surface, might introduce errors of unknown magnitude. For 
this reason, on further reflection, I think it better to have a perman- 
ent cover on micrometers intended for use with high power object- 
ives, and to have the corrections of such micrometer determined with 
such cover in situ. 
This leads me to notice a table of measurements some time since 
published by Mr. Chas. Fasoldt, which I suppose was intended to 
invalidate the result of the investigation of Centimeter Scale “A” and 
its so-called copies, by the different observers who have investigated 
them. Mr. Fasoldt does not in his published paper give sufficient 
data to enable one intelligently to criticise or judge of the accuracy 
of his work ; but there is one element of uncertainty about it that 
seems quite patent, viz: that it does not appear that the glass disc 
upon which the lines were ruled had either surface plane or that the 
two surfaces of the said disc were parallel. 
If nothing else appeared, to my mind the fact that the space 
was measured with different sorts of illumination, and with the 
