384 



THE AMERICAN NATURALIST [Vol. LV 



The advantage of this type of needle control over the sliding 

 motions of the Barber pipette holder lies in greater ease and 

 convenience of manipulation as well as in greater refinement of 

 needle control. The simplicity of the apparatus is such that its 

 manipulation requires no great degree of technical skill, and in 

 its construction have been avoided as far as possible all elaborate 

 and complicated adjustments. The screw heads turn easily 

 enough to be rolled under pressure of one finger each. The com- 

 plete apparatus for holding two needles may be made to occupy 

 a three-inch cube, and clamped close under the nosepiece; five 

 of its six screws can be operated with one hand, leaving to the 

 otlier, the mechanical stage ratchets and the sixth needle screw 

 adjacent to these. The needles may be held so close to their 

 points that no vibration is noticeable, especially since no moving 

 part is handled during manipulation. The needle points move 

 in arcs of circles, but since the ratio of length of arc to radius 

 is small, tliese are virtually straight lines, in the plane of, and 

 perpendicular to the plane of the focus of the objective. 



A more detailed account of the working of this instrument, and 

 a description of various devices that may be added to facilitate 

 needle manipulation, will be included in a subsequent paper deal- 

 ing with the results of its application to the study of proto- 

 plasmic activity. 



Geo. H. Bishop, 

 C. E. Tharaldsen 



