NORMAL SULPHATES OF POTASSIUM, RUBIDIUM, AND CESIUM. 
471 
thermometer to it and gently drawing the latter aside, was removed from the chamher 
and transferred to the thickness measurer. The greatest care was taken not to 
disturb the positions of the crystal and compensator on the table of the tripod. The 
large cover-glass (cover-wedge of the previous memoir) was too large to be taken 
through one of the windows of the chamber along with the tripod, so was left inside, 
being raised with the left hand while the tripod was removed with the right to the 
nearest resting place, the top of the air-bath ; the cover-wedge was then turned over 
so as not to injure the silver reference ring and laid on the floor of the chamber. The 
tripod was then removed with both hands to such a position on the glass base of the 
thickness measurer that the agate pointed end of the measuring bar would fall exactly 
on the centre of the compensator, over which the silver reference ring of the cover- 
wedge had been situated during the observations. The height of the plane of the 
tops of the three tripod screws at this point was then first determined Ijy laying on 
the screws a large circular disc of glass similar to the cover-wedge, and whose surfaces 
were truly plane and the thickness of which had previously been repeatedly deter¬ 
mined at a position near the centre which was conveniently indicated by a small 
internal bubble. The disc was laid so that the bubble was over the centre of the 
compensator. The measurement was then made by lowering the counterpoised bar 
into gentle contact with the top of the disc and reading the scale with the aid of the 
micrometer. This height, minus the known thickness of the disc, gave the height of 
the plane of the tops of the screws. The disc was then removed and the bar lowered 
down upon the compensator, and the height again noted. The difference between 
this and the height of the screws gave the thickness of the air film. The bar was 
again raised and the compensator next removed, without disturbing the ciystal, a 
matter requiring some nicety of manipulation with a pair of small ivory-tij^ped 
forceps ; the bar was then allowed to fall gently on the crystal, when another 
measurement was taken. The difference of this and the last was of course the 
thickness of the compensator, as nearly as possible 5’250. It then only remained to 
once more raise the bar, remove the crystal, allow the bar to fall on to the table of 
the tripod, and take a final measurement of the height of this. The difference 
between this reading and the previous one afforded the measure of the thickness of 
the crystal. In cases where the three-point method was employed the only difference 
was to determine the mean height of the three particular table points used, with the 
aid of a small disc of glass, of known thickness at the centre, and placed on the same 
points, instead of determining the height of the surface of the table itself The 
length of the screws was evidently afforded by subtracting from the height of the 
screws the height either of the table itself or of the points, according to the method 
of supporting the crystal employed. 
The four desired basal quantities, the thickness of the crystal, the thickness 
of the compensator, I the length of the screws, and d the thickness of the air-film, 
were thus determined exactly along the vertical line passing through the centre of 
