MICROMETRY The measurement of minute divisions of space. The best known device for obtaining micro metrical measurements in microscopy and photomicrography is the stage micrometer ruled into lines to rya or o in., or and mm. A fairly accurate estimation of the size of an object can be made by focusing the ruled lines of this micrometer on the stage and carefully noting the space in the field taken up by a certain number of lines. The object to be measured is then substituted for the micrometer slip and focused with, of course, the same objective, eyepiece, and length of draw-tube. The approximate size of the object can then be gauged by comparing the diameter of the specimen with the space previously taken by the ruled lines. For accurate work a micro meter eyepiece is necessary to measure off the rulings of the stage micrometer. In the micro meter eyepiece a small plate of equidistant ruled lines is fixed in the focus of the eye lens of the eyepiece, the lines being then visible when the microscope is in use. The stage micrometer is focused in the usual way, and the number of its lines which take up the same space as a certain number of the eyepiece divisions is noted. The stage micrometer is then replaced by the specimen, and the diameter of the latter is compared with the previously noted eyepiece divisions. It is not necessary to have fixed numbers of lines per inch in the eyepiece rulings ; in fact, two points in the focus of the eye lens are all that is required. Two small
ink marks on a cover slip will answer the purpose. The cover slip is dropped on to the stop of the eyepiece and the number of rulings of the stage micrometer which fill the space between the ink spots noted ; the size of the object can then be calculated.
For extreme accuracy the mechanical-screw eyepiece micrometer is necessary. In this eyepiece are two wires, one of which is fixed while the other, worked by a travelling screw turned by a milled head, traverses the field. Across the field is fixed a plate provided with minute teeth, and each revolution of the milled head moves the wire from one tooth to the next. By this means the stage rulings can be compared with the distance between the two wires with extreme accuracy.
In photomicrography the magnification of an object is easily found without the aid of a ruled eyepiece. The object is first focused on the ground glass of the camera, and its diameter marked by ink spots. The stage micrometer then replaces the object and, after focusing, the number of lines between the ink spots is read off. This gives the size of the object, and the magni fication is then easily calculated by measuring the distance between the two ink marks.