10 PROFESSOR C. V. BOYS ON THE 
are used to protect the freely hanging mirror from draughts when observations are 
made upon it with the cathetometer. The window, fig. 11, is made of biass, electro- 
gilt, with a small aperture just large enough to allow the telescope T, figs. 18, 19, 
and all parts of the scale §, figs. 18, 21, to be seen from all parts of the mirror. The 
outer face of this window is covered with a plate of glass optically worked by H1temr, 
held in place by soft wax. The top and bottom sectors and the faces F F are smeared 
with vaseline to make an air-tight joint when the window is in position. The 
window, fig. 12, is made of brass, electro-gilt ; it is similarly fixed in position behind 
the mirror. A brass tube, lightly filled with cotton-wool, screws into this window on 
one side. The window shown in vertical section, fig. 13 and in plan, fig. 14, is made 
of brass with a flat tubular opening with rounded ends. This enters the rectangular 
chamber and rests against the faces F F, which have been cut away at their lower » 
part sufficiently for this purpose. The inner end of this tube is covered with a 
naturally cleaved thin film of mica, which enables the two quartz fibres hanging from 
the freely suspended mirror to be seen by two high-power microscopes whose noses 
penetrate into the flat tube without allowing them to be blown about by draughts. 
The use of mica for this purpose is essential. Mr. CunyncHame had previously 
shown me that the definition of a good telescope, which is absolutely destroyed by 
window glass held im front of it and impaired by any but perfect optically worked 
glass, is not affected by a leaf of mica, even though it may be bent or be apparently 
irregular. In the same way, the apparent position of anything seen by a microscope 
is altered if a piece of ordinary cover glass is placed between the two at some distance 
from the object, besides which the definition suffers. A thin leaf of mica in no way 
affects the definition or the apparent position, and so the distance apart of the fibres 
measured by the microscopes, as will be described later, is the true distance, which it 
never would be if cover glass were employed. This distance must be measured with 
the mirror freely hanging so that it may be the same as it is when the deflections, etc., 
are being observed. 
Resting on the base B, fig. 1, are four india-rubber discs I R, with large central 
holes, their object being to form a soft cushion for the lead balls MM to rest upon 
when not suspended or to fall upon in case of accident. 
In the same way I have provided a safety catch and recovering device in case the 
small balls should fall down the central tube. When the mirror is suspended and 
has been adjusted with its torsion fibre axial, the loss of time that would ensue if 
the little balls could only be recovered by moving the central tube is so great that 
some contrivance of the kind is necessary. At first I merely had some cotton-wool 
at the bottom of the tube, and fished for the little balls with an india-rubber tube 
let. down through one window opening. On sucking air through the end with the 
mouth, the balls could generally be picked up and drawn out attached to the lower 
end of the pipe. My present plan is less precarious. W is a piece of wood loosely 
fitting the tube. On this there is half an inch or so of cotton-wool on which is a dise 
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