Notes on the Structure of Ophioglossiivi. 191 



Finally, while rotating in the lathe, the paraboloid is perforated 

 through the axis with a steel drill and turpentine. 



Paraboloids can be made true enough for most purposes, if 

 finished as above described, but if great accuracy is a desidera- 

 tum, the figure may be corrected after the rough turning, by 

 means of the following appliance. 



It is a property of the paraboloid that the face of every sec- 

 tion taken parallel to the axis, is an exact counterpart, and, in 

 form, is the same parabola. This enables us to verify and cor- 

 rect the figure. From the further end of a base-board, clamped 

 to the bed of the lathe, hinge a piece of board about two inches 

 wide. Let this be so adjusted that when the front edge is 

 raised the upper plane of the board falls exactly parallel with 

 the lathe centers. Rough file out a piece of sheet brass, some- 

 thing like the template, to serve as a grinder. Lay this on the 

 face of the hinged piece of wood, and press it up on the revolv- 

 ing glass, smeared with fine emery and water. After a few 

 turns lower the board and shift the brass grinder endways to 

 another position, either in or out. Repeat this continually, oc- 

 casionally turning the brass over in order to equalize the sides. 

 By this operation the parabolic figure of both the grinder and the 

 glass will soon correct each other; of course, a piece of the swing 

 board must be scooped out sufficiently to admit nearly half the 

 paraboloid. 



In accordance with the above mode of procedure, the para- 

 bola is originated and its size pre-determined by the given 

 focal distance. The ordinary dry parabolic illuminator is usu- 

 ally made about \ inch focus; for an immersion ^-^ will do bet- 

 ter; but if this is to be used as an an animalcule holder, ^ 

 will be found sufficient. 



NOTES ON THE STRUCTURE OF OPHIOGLOSSUM. 



BY MARK W. HARRINGTON, M. A., F. L. S. 



{Received December i6th, i8yS.) 



During the winter of 1876-7, I had the opportunity of study- 

 ing, in the botanical laboratory of Professor Hofrath Schenck, 

 of Leipzig, the structure of O. pediinculosum, Desv., and of com- 

 paring it with the structure of several other species of the 

 genus. I compared my results at each stage with those ob- 



