ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 129 



turned in a lathe. A projecting crystal of the diamond caught these 

 spirals and carried them unbroken to the end of each line, leaving them 

 a tangled mass of threads. Even after they were protected by a cover- 

 glass cemented to the surface, many of these spirals remained intact. 

 Judging by the diflference in focus of the various parts, the height of the 

 mass, before the plate was covered, must have been 1/500 of an inch. 



The same ruling crystal may produce smooth lines or either chips 

 or threads, according to the motion of the diamond, as may be seen by 

 examination of the accompanying rulings In these plates one-half 

 of the lines of the bands are ruled by a forward motion and one-half 

 by a backward motion of the diamond. Chips may be formed in 

 ruling bands of very fine lines, as illust fated in the bands of lines 

 24,000 to the inch. 



It must not, however, be supposed that lines of the best quality 

 always present the appearance described above. While it is exceed- 

 ingly rare that lines appear as well after the surface of the glass has 

 been rubbed as before, many instances have occurred within my 

 experience in which the difference, especially in fine lines, was not 

 particularly noticeable. According to the limited evidence at hand, 

 the coarser lines of Nobert's bands present some of the characteristics 

 which I have described. I have restored two of these plates, in which 

 the lines had become nearly obliterated by some kind of condensation 

 under the cover-glass. In one the quality of the lines was not much 

 affected by the operation of cleaning, but in the other the dark gloss 

 which characterizes the heavy lines of nearly all of Nobert's plates 

 was entirely destroyed. The finer lines, however, were much less 

 affected than the coarse ones. 



Lines of the character thus far described are evidently unsuited 

 to the ordinary work of the microscopist. It is my experience 

 that lines which are the most symmetrical in form and the most 

 beautiful in appearance are produced indirectly rather than by 

 the direct action of the diamond in cutting a groove in the glass. 

 They can be protected to a certain extent by a cover-glass, but 

 they are liable to undergo changes which will affect their original 

 structure. Except for purposes of investigation, therefore, there is 

 no advantage to be gained by ruling lines of this character. Three 

 conditions must be fulfilled in the production of lines having a per- 

 manently good character : — 



1. The glass must be tough. There is a marked difference in the 

 character of the filaments produced, and, to a certain extent, of the 

 lines themselves, yet the conditions under which the lines in the series 

 of plates illustrating this paper were ruled were the same in nearly 

 all of the plates — i. e. the same diamond was used, its setting re- 

 mained unchanged, and there was no change in the pressure of the 

 diamond upon the surface of the glass. I may add also, that I 

 have in my collection several other plates which were ruled espe- 

 cially to test the question of the requisite quality of the glass. 

 They all agree in giving evidence that glass of a given quality will 

 always yield lines of nearly the same quality — the ruling crystal 

 remaining the same and in the same position. 



Ser. 2.— Vol. IV. K 



