14s ./. M. Blake — Method of JRufrmg Used by JVbbert. 



tance apart from the beginning to the end of the selected time- 

 interval, and they would afford the data for calculation and 

 measurement by which the final working electric contact-points 

 could be located. Success would depend upon the possibility 

 of repeating the temperature conditions, provided the chrono- 

 graph worked with precision. 



The ruling point is a vital part of the apparatus. Mr. 

 liogers' experience tended strongly to the use of ground and 

 shaped points and edges. It is expected, however, that with 

 the light pressure needed in ruling these short bands covering 

 a very small area, and with the pressure controlled by a very 

 light spring, that a suitable working-point might be selected 

 from minute chance-broken fragments of diamond. A very 

 delicate point might remain intact for a long time since no 

 heavy moving parts would be involved, as in a ruling machine. 



The errors in spacing and the curvature of the lines on a 

 Robert diffraction grating were revealed by placing two photo 

 copies so that the film-surfaces were in contact, and the lines 

 on the two made nearly parallel. Shaded irregular bands were 

 thus produced indicating the errors in spacing. The writer 

 has described this method in this Journal, July 1874. 



These errors had apparently no periodicity as would be 

 expected to result from the screw motion on a ruling machine 

 of the ordinary type. It also became apparent that the lines 

 were curved, and it was estimated that the radius-bar or spring- 

 pole holding the ruling point was approximately eight feet in 

 length. 



We can see that this gives an insight into Nobert's methods ; 

 and the inference is that his most delicate ruled bands did not 

 require the use of a screw or a sliding mechanism. The curva- 

 ture of lines shown in his rulings would result from the ruling 

 point being carried by a spring. The diminishing depth of 

 ruling as the closest lines are approached may have been accom- 

 plished by placing the ruled surface in an oblique position, 

 rather than by a diminished magnetic pull on the spring carry- 

 ing the diamond. 



The indications are that in ruling his finest bands of lines, 

 Xobert depended entirely upon changes of temperature through 

 measured time-intervals to give the required spacing. 



