GEODETICAL STANDARDS WITH THE ENGLISH STAND AEH YAED. 
165 
side: the plate is rather more than an inch thick. Towards the apex of the projecting 
triangle there is a circular hole 2 inches in diameter through the plate, through which 
the gun-metal cylinder holding the microscope passes. Equidistant from the centre of 
this hole, and equidistant from one another, are three vertical screws strongly bolted into 
the iron plate and projecting upwards about 2 inches in length. These screws pass freely 
through the holes in the arms of the gun-metal cylinder. Suppose now two nuts run- 
ning upon each of these screws, one above and one below each of the arms, and it is 
clear that we have the means of rendering truly vertical the axis of the microscope, and 
also of holding the microscope very firm by clamping down the upper nuts. It will also 
be seen that the microscope is held without the least strain, and that it can be raised or 
lowered small quantities so as to bring to focus over a given object. The iron plate is 
not held down to the stone in any way ; its own weight gives sufficient stability ; it may 
he shifted to any position on any of the stones. 
Illumination . — Much depends upon the proper illumination of the divided surfaces 
under observation. A candle, whose flame is mechanically kept in a constant position, 
stands behind the microscopes ; and its light, condensed by a lens 3 inches in diameter, 
passes through an aperture in the projecting part of the cast-iron plate, being brought 
to a focus on the divided surface under observation. Abundance of light is thus 
obtained, and the candle being above the bar, the heated air is continually carried away 
from it ; besides, the heat of a candle is the least practicable with a sufficiency of light*. 
Carriages . — The box containing the bar, or bars under observation, is supported by 
two carriages which run upon the rails that have been described as fixed on the upper 
surface of the large mahogany beam ; one of the rails is flat, the other triangular in 
section ; each carriage runs on three wheels, two of which, being grooved, run on the 
angular rail, the third on the flat rail. Thus it will be seen that the motion of the 
carriage is without any possible jamming. Each of the two carriages is double, that is, 
consists of an upper and lower carriage ; the upper carriage runs upon short rails on the 
surface of the lower carriage, and in a direction perpendicular to the motion of the 
latter. A slow-motion screw affords the means of communicating, when required, a small 
motion to the upper carriage. Without going into further details as to the construction 
of these carriages, it may be sufficient to say that the different parts are so put together 
that no shake exists, nor can it be introduced by wear. The box containing the bar or 
bars under comparison has therefore, when resting on the carriages, a perfectly steady 
bearing ; while it can be moved in a longitudinal direction by the running of the carri- 
ages along the rails on the mahogany beam, or moved transversely by the movement of 
the upper carriages on the lower. 
* This method of illumination was decided on after a considerable number of experiments with gas, oil lamps, 
&c., directed and condensed, or reflected in different ways. Had there been only a few different lengths to 
compare, the light might have been brought in from the outside of the room through horizontal holes in the 
wall and piers, and in this manner some of the earlier comparisons on different ten-foot bars were made. 
But this method could not^ have been applied to the comparisons generally, on account of the large number of 
holes that would have been required. 
