154 



MEMORANDA. 



" the measurements made by it are by no means so delicate 

 as they appear to be." In taking a unit, from which to con- 

 struct the scale, a stage micrometer must be employed, and on 

 the accuracy with which this is graduated depends, of course, 

 the exactness of the subdivisions effected by means of the 

 screw. This objection applies equally to all eyepiece-micro- 

 meters ; but the screw-instrument has the positive disad- 

 vantages of being constructed of parts very apt to become 

 deranged, and capable of being replaced by none but a first- 

 rate workman. The effects of friction cannot be wholly 

 obviated ; the screw is apt to wear, and to wear unequally ; 

 and the uniformity of all its parts, — even when it leaves the 

 workman's hands, — may be reasonably suspected. The price 

 is necessarily so high as to preclude its general employment 

 by those engaged in microscopic observations. 



In Henle and Pfeifer's ' Zeitschrift fiir Rationelle Medicin' 

 (band X. heft 1), Hermann Welcker, a medical student at 

 Giessen, proposes a new kind of micrometer, capable of 

 furnishing indications of extreme delicacy, and in elegance 

 of principle and cheapness far surpassing the cobweb screw- 

 micrometer. 



The following description will enable any one familiar with 

 the elementary principles of trigonometry to comprehend 

 the mode of constructing and using such an instrument : — 



Construction. — Across the stop of an ordinary negative eye- 

 piece, two very fine threads, from a small spider cocoon, are 

 stretched at right angles to each other, and, by means of a 

 little copal varnish, are fixed in such a position that the 

 shorter intersects the longer thread, cutting off about one 

 quarter of its length. The relative position of the threads is 

 shown in Figs. II. and III., where they are indicated by the 

 letters A B and C D. To the upper part of the tube of the 

 microscope is fixed transversely a brass plate, along which 

 plays a pointer, firmly attached to the eyepiece immediately 

 beneath its milled rim. The appearance of this apparatus is 

 shown on a reduced scale in Fig. I. Upon the edge of the 

 brass plate is drawn an arc of a circle concentric with the 



Fig. I. 



eyepiece, and this arc is then subdivided into degrees, and 

 any fractional parts which may be required. 



