128 SUMMARY OF CURRENT RESEARCHES RELATING TO 



marked characteristics of lines of good quality ruled upon glass, 

 and to illustrate these characteristics by corresponding specimens. 

 To one who is familiar with Nobert's bands a perfect line need not be 

 described. It is densely black, with at least one edge sharply defined. 

 Both edges are perfectly smooth. Add to these characteristics a 

 rich black gloss, and you have a picture of the coarser lines of a 

 perfect Nobert plate. How are those lines produced ? In the study 

 of the action of a diamond in producing a breaking fracture in glass 

 the Microscope seems to be of little service, but we can call it to our aid 

 in the study of its action in ruling smooth lines. One would naturally 

 suppose that a line of the best quality would be produced by the 

 stoppage of the light under which it is viewed by the opaque groove 

 which is cut by the ruling diamond. "Without doubt this is the way 

 in which lines are generally formed. But it is not the only way in 

 which they can be produced. An examination under the Microscope 

 will reveal the fact that in some instances at least, a portion of the 

 glass is actually removed from the groove cut by the diamond ; and 

 that the minute particles of glass thus removed are sometimes laid up 

 in a windrow beside the real line, as a plough turns up a furrow of soil. 

 On the finest plate I have ever produced every line remained in perfect 

 form for about two months. I then first noticed a tendency on the part 

 of some of the single lines to disintegrate, while the lines ruled in 

 closer bands seemed to retain their good qualities. This disintegration 

 finally became so marked that, as an experiment, I removed the cover 

 and cleaned one-half of the surface of the glass by rubbing with 

 chamois skin. The difference in the appearance of the two halves is 

 now very marked. Above, the dense black lines remain. Below, a 

 ragged abrasion of the surface of the glass has taken place. Above, 

 the furrowed lines as originally formed are preserved ; below, there is 

 a coarse scratch. It may be said that the action in this case is acci- 

 dental and abnormal. In reply, I can say I have prepared plates 

 which show that the particles of glass removed take four characteristic 

 forms, (a) They appear as chips scattered over the surface of the 

 glass. (6) They appear as particles so minute that when laid upon a 

 windrow and forming an apparent line, they cannot be separated under 

 the Microscope, (c) They take the form of filaments when the glass 

 is sufficiently tough for them to be maintained unbroken, (d) They 

 take a circular form. 



I regret that three of the most striking specimens were broken in 

 mounting. In one a perfect line about 1/30,000 of an inch in 

 width was formed with a clear space between it and the groove cut by 

 the diamond. There was not a single break in these filaments from 

 beginning to end, but at nearly equal intervals of about 1/100 

 of an inch half-knots were formed similar to those formed in a 

 partially twisted cord. By rubbing the surface of one end these fila- 

 ments were broken up. For the most part they assumed a semi- 

 circular form, but some of them maintained their thread-like form 

 and became twisted together in the most intricate fashion. 



In the third specimen, which was broken in mounting, the glass 

 removed took a spiral form like the spiral chips from steel when 



