[Illustration: Fig. 165.--Maltwood’s Finder.] Dr. Pantacsek’s finder appears to have some advantage over Maltwood’s, but it cannot be used with the same facility, and therefore will not displace an old favourite. _The Amyot_ finder I have long had in use; it is efficient and inexpensive--can indeed, if misplaced or lost, be replaced by the aid of the square and compasses. [Illustration: Fig. 166.--Amyot’s Object Finder.] _The Okeden_ finder consists of two graduated scales, one _vertical_, attached to the fixed stage-plate, the other _horizontal_, attached to an arm carried by the intermediate plate; the first of these scales enables the worker to “set” the vertically-sliding plate to any determinate position in relation to the fixed plate, while the second gives the power of setting the horizontally-sliding plate by that of the intermediate. _Micrometers._--It is of the utmost importance to have a means of measuring with accuracy the objects, or part of objects, under observation. The most efficient piece of apparatus for the purpose is the micrometer eye-piece, the earlier form of which, Jackson’s, has been described under the heading _Eye-pieces_ (p. 144). In the case of micrometers, as in that of most other accessories, every optician has his own adaptation and method of employing the same. For the measurement of bacteria, a stage micrometer should be used with a camera lucida. The stage micrometer consists of a slip of thin glass ruled with a scale consisting of tenths and hundredths of a millimetre. The image is projected on to a piece of paper placed on the table, and the drawing made, and the object to be measured can be readily compared with the scale. [Illustration: Fig. 167.--The Ramsden Micrometer Eye-piece.] In the Ramsden micrometer eye-piece, as previously explained, two fine wires are stretched across the field of an eye-piece, one of which can be moved by a micrometer screw. In the field there is also a scale with teeth, and the interval between them corresponds to that of the threads of the screw. The circumference of the brass head is usually divided into one hundred parts, and a screw with one hundred threads to the inch is used. The bacterium to be measured is brought into a position in which an edge appears to be in contact with the fixed wire, and the micrometer screw is turned until the travelling wire appears to be in contact with the other edge. The scale in the field and scale on the milled head, together, give the number of complete turns of the screw and the value of a fraction of a turn in separating the wires. In the micrometer eye-piece constructed by Zeiss, the eye-piece with a glass plate with crossed lines is carried across the field by means of a micrometer screw. Each division on the edge of a drum corresponds to ·01 mm. Complete revolutions of the drum are counted by means of a figured scale in the visual field.