188 PROCEEDINGS OF SECTION A. 
due to changes in temperature, or the relief of certain surface 
strains inherent to the glass in its unannealed condition. 
I pass on now to a matter of equal importance with any 
hitherto dealt with, viz., the preservation of the completed 
rulings. Ordinarily in the case of micrometer rulings varying 
from 1 m.m. to .01 m.m. all that is necessary is to fill the lines 
with graphite, and mount the cover on a slip with Canada bal- 
sam. But this method is not suited to the finer rulings, or 
where it is desirable to preserve the lines without the graphite 
filling, as in the case of test plates. Nor is it possible to pre- 
serve them by attaching the cover glass to a cell wall or ring 
of cement or wax, as is frequently done with other micro- 
scopical preparations. I myself tried every, or almost every, 
known cement and wax cell at all suited to the purpose, and in 
every instance it was only a question of time, probably a year or 
more, and the cover glass became coated or covered with minute 
crystals in some instances, or microscopical beads of moisture 
in others, to such an extent as to detract greatly from the beauty 
and perfection of the lines, and in some cases to partially ob- 
literate the finer bands altogether. It, therefore, remained for 
me to endeavour to mount the ruled plates in a medium pos- 
sessing a refractive index differing from glass by an amount 
equal to the difference between glass and air. Several such 
media existed, and had been used for other purposes, but with 
only partial success. These were prosphorus, sulphur, and 
realgar, or arsenic disulphide. The latter appeared to me the 
most promising substance to work with, seeing it possesses a 
refractive index equal to 2.549, but its use is attended with 
many difficulties, and J worked with it for nearly a year with 
only partial success. I soon abandoned all attempts to use it in 
a liquid form dissolved in the usual solvent, bromine, which I 
found both uncertain and dangerous to use, and turned my at- 
tention to the production of thin films by sublimation. With 
these I was more successful, and after a time was able to pro- 
duce exceedingly thin films, which have so far proved quite per- 
manent. Some of the films here shown have been mounted over 
two years, while those sent to London some little time ago with- 
stood all the changes of temperature to which they were sub- 
jected on the journey without showing any signs of depreciation. 
As tc the general results I have so far achieved, I cannot do 
better than quote from the presidential address of the president 
of the Royal Microscopical Society for last year, to whom 
samples of the various rulings were submitted for critical ex- 
amination, the quotation being as follows:—‘“ You have re- 
ceived a valuable present from Mr. H. J. Grayson, of Melbourne, . 
of micrometric rulings, mounted in a high refractive medium, 
the refractive index of which is as great as that of a diamond, 
viz., 2.549. This subject is of so great importance that special 
