in Ruling his Test-Plates. By William A. Bogers. 89 
exceedingly sharp knife-edge may be formed, possessing excellent 
ruling qualities. Moreover, in ruling heavy lines for diffraction 
plates, the cutting edge retains its form for a long time. In setting 
the diamond for ruling, the natural face should be slightly inclined 
to the surface to be ruled. The Brazilian " bort " seems to give 
the best and most durable cutting edge. With a diamond prepared 
in this way, the Hne formed by the intersecting faces being about 
^\ of an inch in length, I find little trouble in ruHng from 60,000 
to 80,000 lines to the inch. 
IV. A few months since Mr. K. C. Greenleaf, of Boston, placed 
in my hands a Nobert plate which had been entirely spoiled by the 
introduction of some kind of fluid between the ruled glass and the 
slide on which it was mounted. Mr. Greenleaf requested me to 
undertake the restoration of this plate, kindly offering to assume 
all the risk of failure. The cover, which had been imperfectly 
cemented to the slide with something like opal cement, resisted 
every attempt at loosening. As a last resort, two pieces, about 
tV of an inch square, were cut with a diamond from the centre 
of the cover- glass. After several trials, one of these pieces was 
cleaned and remounted without material injury to any of the 
bands. The 19 th is quite as easily resolved as in other Nobert 
plates. 
The other piece, being less perfect, was made the subject of a 
somewhat careful study. Among other experiments, an attempt 
was made to fill the lines with graphite ; but it was found impos- 
sible to do so. Even the coarsest lines would not receive and hold 
it. As I had never before found any difficulty in filling lines either 
coarse or fine, this result, so entirely unexpected, was noted down 
as one of which no explanation could be given at that time. 
A few weeks afterwards, I succeeded in reducing a black carbon 
to a knife-edge. Upon an examination of the first lines ruled with 
it, two facts at once engaged my attention. First, the Hues were 
finer and smoother than any I had ever before ruled. They pos- 
sessed that quality of glossy blackness which characterizes nearly 
all of Nobert's lines. Moreover, they seemed to stand out more 
boldly in perspective than lines ruled with the ordinary diamond. 
Everyone who has made a study of Nobert's difiraction lines will 
at once recognize this boldness of perspective as a characteristic 
feature. Secondly, I was equally surprised to find that the lines 
would not receive and hold graphite. 
As these results were confirmed by further observations, it did 
not seem too much to say that possibly the secret of Nobert's suc- 
cess might consist in his use of a prepared carbon. The natural 
stone is entirely unfit for ruling purposes. 
But it appeared subsequently that this conclusion was too 
hastily formed, as far as the capability of receiving graphite is con- 
