98 



E. M. NELSON ON MICROSCOPE CONSTRUCTION. 



an idle nut to butt against a stop ; if this nut receives damage or- 

 strain to its thread it is of no importance. The first kind adopt 

 a continuous motion in order to secure immunity from this 

 danger, and put up with the great disadvantage of having a fine 

 adjustment which does not follow the direction of the movement 

 of the milled head. 



The following simple device has been designed to effectually 

 prevent any damage taking place. To the right hand side of the 

 limb, where the micrometer drum-head is placed, a short piece of 

 tube, threaded on the outside, is fixed, and through it the fine 

 adjustment pinion passes just like the cannon pinion in a clock. 



Fig. 1. 



Fig. 2. 



An idle nut works on this screw in a slot inside the micrometer 

 drum. It is then arranged that this nut will permit ten rotations 

 of the fine-adjustment pinion to be made, and then stop further 

 motion by butting either against the side of the limb or against 

 the end of the inside of the micrometer drum. Figs. 1 and 2 will 

 make this simple device clear without further explanation. 



NOTE ON PLEUROSIGMA ANGULATUM. 



By Edward M. Nelson, F.R.M.S. 



{Read January 28th, 1913.) 



About the end of the eighties I took a photomicrograph of a 

 specimen of Pleurosigma angulation, which had been broken in 

 a very remarkable manner so that it was possible to demon- 



