G. GENEKAL NATURAL HISTORY AND ZOOLOGY. 269 



however, against any disturbance, a second magnet holds the 

 head of the screw in place while the first one is moving back. 

 By replacing the lower stop by a wheel revolving with the 

 screw, the latter can be made to correct its own errors by 

 slight changes made in the periphery of the wheel. As to 

 how Nobert rules his lines, Mr. Rogers thinks it quite evi- 

 dent that it is done with a diamond having a knife-edge ; 

 and he details the method employed by himself in obtaining 

 a suitable edge. He states that certain kinds of glass wnll 

 receive perfect lines only in one direction, the glass having a 

 certain grain, as he calls it. Perhaps tliis is not unconnected 

 with the "axis of rigidity" observed by Dr. Draper in con- 

 structing his glass specula for telescopes. To avoid errors 

 from false lines, and to obtain greater distinctness, the fine 

 lines (ruled between two heavy finding lines) should be filled 

 with grajihite, and Mr. Rogers states that the graphite of 

 the New York Graphite Company will easily fill the finest 

 line that can be ruled with a diamond. When the lines are 

 thus filled, they appear with their true width and with dis- 

 tinct edges. The result of Mr. Rogers' experience in ruling 

 with a knife-edge diamond is that its ruling qualities im- 

 prove with use, under certain conditions. A diamond that at 

 first would barely rule 10,000, afterward successfully ruled 

 bands 50,000 to the inch ; this does not hold good, how- 

 ever, of the heavier lines required for diffraction plates. The 

 sharp, singing sound of a perfect ruling indicates to a prac- 

 ticed ear the nature of the cut almost as well as the sisrht. 

 In attempting to fill the lines of an injured Nobert test-plate 

 with graphite, Mr. Rogers found it impossible to do so. Sub- 

 sequently he ascertained that lines ruled by himself, but with 

 a black carbon, not only were finer and smoother than any 

 that he had ruled before, but that they would not receive 

 and hold graphite. He therefore concluded that Nobert's 

 success might consist in his use of a prepared carbon, a con- 

 clusion too hastily formed so far as graphite is concerned, for 

 afterward he found that lines ruled with the same carbon, 

 but held at a different inclination, would receive the graphite. 

 The best results, however, have been obtained with the pre- 

 pared carbon ; though it is capricious in its action, it is much 

 harder than any other kind of diamond. Proceedings of the 

 America)! Academy of Arts and Sciences. 



