PROCEEDINGS OF THE SOCIETY. 377 



Dr. C. V. Zenger's letter was read, describing a new mountiag 

 medium, consisting of tribromide of arsenic in bisulphide of carbon, 

 and giving a refractive index of from 1 • 6696 to 1 • 7082. An improved 

 slide for viewing the object on both sides was also described. 



Mr. C. H. Hughes' description was read of a stage for use with 

 high powers, to prevent the decentering of the condenser, especially 

 when used with immersion contact. Vertical, horizontal, and oblique 

 motions are given to the slide while the stage remains stationary, but 

 can be rotated. 



Mr. J. Mayall, jun., described the original ruling machine of 

 the late Herr F. A. Nobert (now the property of Mr. Crisp), which 

 was exhibited to the meeting. He said he should not enter minutely 

 into the details of the construction, as they could hardly be under- 

 stood without a close insijection of the machine, many portions of 

 which were too delicate and complex to be handed round to the 

 meeting. The foundation of the machine was a dividing engine 

 calculated to produce parallel divisions far finer than could be 

 marked by any ruling point yet discovered. The division-plate 

 had 20 circles of " dots," and these were supplemented by extremely 

 fine graduations on two bands of silver imbedded near the edge, 

 which were viewed by means of two compound Microscopes, each 

 provided with eye-piece screw-micrometers of special construction. 

 The movement of rotation was effected by a fine tangent screw 

 acting upon a worm on the vertical edge of the division-plate. The 

 tangent screw was controlled by a large milled head about 4 in. in 

 diameter, and a graduated drum showed the amount of motion. The 

 method employed by Herr Nobert for obtaining the minute divisions 

 of his test-plates (ranging from 1/1000 to 1/20,000 of a Paris 

 line) was to convert the radius of the division-plate into a lever to 

 move the glass plate on which the rulings were made at right angles 

 to the motion of the ruling point. For this purpose, he attached 

 to the centre of the rotating division-plate a bent arm on which 

 slided a bar of silver, having at one end a finely-polished steel point 

 which could be adjusted by a scale and vernier so as to project more 

 or less beyond the centre of the division-plate or axis of rotation. 

 The radius of the division-plate thus became the long arm of the 

 lever, whilst the radius of the projection of the polished steel point 

 beyond the axis of rotation formed the short arm, the centre of the 

 division-plate being the fulcrum. The motion of the short arm of 

 the lever was communicated by contact with an agate plate to a 

 polished steel cylinder adjusted to slide at right angles to the move- 

 ment of the ruling point in V-shaped bearings of agate. The steel 

 cylinder carried a circular metal table on which the glass plate to be 

 ruled was fixed by wax and clamps. The motion of the lever arms 

 was, of course, in arc, and hence the divisions would not be strictly 

 equidistant unless compensation were made for the difference in 

 length of the arc and its sine; but since the actual space in- 

 cluded between the first and last lines of Herr Nobert's test- 



