90 On a Possible Explanation of the Method employed by Nobert 
cerned. During all these observations, the position of the diamond 
in its holder remained unchanged ; but it was afterwards found that, 
by giving it a certain inclination with respect to the surface of the 
ruled plate, it was possible to rule lines, both coarse and fine, which 
would receive the graphite in the most perfect manner. In general, 
however, lines ruled with a carbon will take the plumbago perfectly 
but once. If they are filled and • the surface of the glass is after- 
wards cleaned by rubbing, it is not possible to fill them equally 
well again. As the filling is not disturbed by mounting in balsam, 
the better way is. to clean the glass thoroughly before ruling, and 
then mount permanently after the first filling. 
Though the carbon is reduced so perfectly to a true knife-edge 
that the intersection of the two faces appears as a line when examined 
with an eye-piece of high magnifying power, it is apparent, never- 
theless, that the cutting edge is composed of distinct and separate 
crystals ; for in many cases two lines have been ruled at the same 
time. Generally one is much coarser than the other. Indeed, by 
regulating the pressure, companion lines can be ruled so fine that 
it is impossible to see them until they are filled. The setting of 
the diamond to rule lines of a given kind and quality is simply a 
question of time and patience. In one hundred trials, perhaps two 
or three may give lines which will receive plumbago, four or five 
may give double lines, and one or two may give lines ' Q^reat 
delicacy. Great care is necessary in the preservation of the ate 
cutting crystal when once found. Notwithstanding the mosc care- 
ful manipulation, it often gives way without visible cause. In 
several instances, I have been able to locate the exact point where 
it was destroyed. • 
In general, the best results have been obtained with the prepared 
carbon. It is, however, somewhat capricious in its action. The 
labour of preparation is also much greater than with the African 
or the Brazihan diamond. The process of grinding occupies from 
five to ten days. That it is much harder than any other kind of 
diamond is conclusively shown by the fact that one specimen in my 
possession has been used in shaping a jewel weighing 180 carats, 
with only a trifling abrasion of its surface. 
In conclusion, I ought to say, in explanation of the somewhat 
incomplete and fragmentary character of this investigation, that it 
has been the gradual outgrowth of experiments undertaken for a 
different purpose. Indeed, whatever has been accomplished thus 
far may be said to be the result of an unsuccessful search after a 
spider that would spin a web suitable for the meridian circle of 
Harvard College Observatory. Failing to find suitable ^' spider 
lines," an efibrt was made to produce on glass, lines of the desired 
quality and size. This was finally accompUshed by etching with 
hydrofluoric fumes ; the lines having been first ruled in a coating 
