88 On a Possible Explanation of the Method employed hy Nolert 
would at first barely rule 10,000. It was only after a service of 
several weeks, its position in the holder meanwhile remaining 
unchanged, that the highest limit named was reached. Four new 
diamonds have since been mounted with precisely the same result. 
It is not to be understood that this remark holds entirely true 
for heavy lines, such as are requisite for good diffraction plates. 
It is the experience of Kutherford and others, that one of the chief 
difficulties in producing such plates is the inability to find a 
diamond which will do its work equally well throughout the entire 
process of ruling. But when only very fine lines are desired, the 
longer the diamond is used, the greater the pressure which can be 
applied without increasing the size of the line. In this way the 
lines can be made much more uniform throughout their entire 
length, than when the diamond barely touches the surface. One 
can hardly say that the diamond sharpens itself by use, but there 
is some evidence that the wear is greater on the two faces than on 
the knife-edge. 
When the diamond does its work perfectly, the cut, even of 
the finest line, produces a sharp singing sound. My ear has 
become so accustomed to this peculiar tone, that I can judge of 
the quality of the lines ruled almost as well by sound as by sight. 
In ruling the highest bands, this sound can be heard throughout 
the entire length of every Hne. It does not always have exactly 
the same character, however, being sometimes much sharper in 
tone than others. 
II. From Mr. Herman, a successful diamond worker of New 
York, I learned a fact which was thought to be of sufficient im- 
portance to justify a somewhat difficult experiment. He stated 
to me that his experience had shown him that the only really 
hard points of a diamond are those where the line formed by the 
intersection of two faces terminates. His directions, therefore, were 
to grind the faces to a knife-edge, exercising great care to leave the 
natural line of intersection untouched as far as possible, and then 
to grind and polish a face nearly at right angles to this line, 
stopping just at its extremity. He assured me that the success of 
the experiment would depend entirely upon neither falling short 
or going beyond this point. Only one diamond has been success- 
fully prepared in this way, and even in this case it is not quite 
certain that this requirement has been met. Its performance is 
sufficiently good to warrant further experiments. 
III. I am indebted to Mr. D. C. Chapman, of New York, for 
a third method of preparing a ruling diamond. It is allowed by 
all familiar with the subject, that the natural face of a crystal is 
harder than any surface formed by breaking the stone into chance 
fragments. By splitting a stone in the direction of a cleavage 
plane, forming an angle of about 40° with this natural face, an 
