THE MEASUREMENT OF MICROSCOPIC OBJECTS. 291 



partly fill the interspaces of the micrometer-scale, can only be 

 approximately effected. Thread-shaped objects should be adjusted 

 to lie across the division- lines at right angles ; the proportion of 

 the interspaces to the thickness of the thread can thus be easily 

 determined within a slight error. Spherical objects should, where 

 possible, be arranged consecutively in a series"; the length of the 

 whole series is then measured and divided by the number of the 

 objects. Similarly, the mean thickness of layers in membranes, 

 starch-grains, &c., is estimated ; and also, where practical, the 

 sectional magnitude of the elementary organs in tissues. 



The screw-micrometer is now almost entirely superseded by the 

 more convenient and much less costly glass micrometer. The 

 arrangement of the instrument is based upon the principle much 

 applied in machinery (e.g., dividing engine, &c.), that the move- 

 ment of a screw is proportional to the angle at which any point 

 of it is turned. The object is placed upon a sliding plate, which 

 is moved backward and forward, by means of a micrometer- 

 screw having a very fine thread (e.g., 5 revolutions to 1 mm.), 

 until the opposite edges of the image coincide successively with 

 a thread extended in the eye-piece. The distance travelled from 

 one point to the other can thus be read off on the graduated 

 screw-head, since the value of one division can easily be calculated 

 from the known value of an entire revolution, and may therefore 

 be regarded as given. 



Another form of this device, well known in England, is the 

 eye-piece screw-micrometer. In this, two parallel threads are seen 

 stretched vertically across the field of view of the eye-piece, of 

 which the one can be made to traverse the field by the micrometer- 

 screw, whilst the other remains stationary. If the edges of the 

 objective-image are now brought to coincide with these threads 

 its diameter can clearly be much more accurately determined 

 than is possible with the help of a micrometer-scale upon glass. 

 The measurement is, however, in this case also only an indirect 

 one ; the relative value of the screw-thread must be specially deter- 

 mined for every objective and for every tube-length, just as with 

 the glass micrometer. 



Upon the principle of Eamsden, of measuring the objective- 

 image instead of the object, is based also the screw-micrometer of 

 H. von Mohl, recently proposed (" Archiv flir mikr. Anat.," Bd. i., 

 1865). Mohl finds the usual kind, where this principle is em- 



