252 PROCEEDINGS OF THE AMERICAN ACADEMY 



was only after a service of several weeks, its position in the holder mean- 

 while remainiug 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 Rutherford and others, that one of the chief difficul- 

 ties 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 increas- 

 ing 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 

 line. 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 importance to 

 justify a somewhat difficult experiment. He stated to me that his 

 experience had shown him that the only really hard points of a dia- 

 mond are those where the line formed by the intersection of two faces 

 terminate. 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 neitlier falling sliort or going beyond this point. Only one dia- 

 mond has been successfully prepared in this way, and even in this case 

 it is not quite certain that this requirement has been met. Its perform- 

 ance 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 



