OF ARTS AND SCIENCES. 253 



angle of about 40*^ with this natural face, an 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 

 f;ice should be slightly inclined to the surface to be ruled. The Bra- 

 zilian " bort " seems to give the best and most durable cutting-edge. 

 With a diamond prepared in this way, the line formed by the inter- 

 secting faces being about -j^ of an inch in length, I find little trouble 

 in ruling from 60,000 to 80,000 lines to the inch. ' 



IV. A few months since Mr. R. C. Greenleaf, of Boston, placed in 

 my hands a Nobert plate which had been entirely spoiled by the 

 inti-oduction of some kind of fluid between the ruled glass and the shde 

 on which it was mounted. Mr. Greenleaf requested me to under- 

 take 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 ^ 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 19th is quite as easily re- 

 volved as in other Nobert plates. 



The other piece, being less perfect, was made the subject of a some- 

 what careful study. Among other experiments, an attempt was made 

 to fill the lines with graphite ; but it was found impossible 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 w'eeks 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 lines were finer and 

 smoother than any I had ever before ruled. They possessed that 

 quality of glossy blackness which characterizes nearly all of Nobert's 

 lines. Moreover, they seemed to stand out more boldly in perspec- 

 tive than lines ruled with the ordinary diamond. Every one who has 

 made a study of Nobert's diffraction 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 

 grapldte. 



As these results were confirmed by further observations, it did not 

 seem too much to say that possibly the secret of Nobert's success might 



