Some makers mount the glass disc together with a circular cover-glass in such a way that when placed in position in any Huyghenian eyepiece of their own manufacture, the scale is exactly in focus for normal vision. Special eyepieces are also obtainable having a sledging adjustment to the eye lens for focussing the micrometer. The value of one division of the micrometer scale must first be ascertained for each optical combination by the aid of the stage micrometer, thus: 1. Insert the eyepiece micrometer inside the ocular and adjust the stage micrometer on the stage of the microscope. 2. Focus the scale of the stage micrometer accurately; the lines will appear to be immediately below those of the eyepiece micrometer. Make the lines on the two micrometers parallel by rotating the ocular. 3. Make two of the lines on the ocular micrometer coincide with those bounding one division of the stage micrometer; this is effected by increasing or diminishing the tube length; and note the number of included divisions. 4. Calculate the value of each division of the eyepiece micrometer in terms of µ, by means of the following formula: x = 10 y. Where x = the number of included divisions of the eyepiece micrometer. y = the number of included divisions of the stage micrometer. 5. Note the optical combination employed in this experiment and record it with the calculated micrometer value. Repeat this process for each of the other combinations. Carefully record the results. To measure an object by this method read off the number of divisions of the eyepiece micrometer it occupies and express the result in _micra_ by a reference to the standard value for the particular optical combination employed. Zeiss prepares a compensating eyepiece micrometer for use with his apochromatic objectives, the divisions of which are so computed that (with a tube length of 160 mm.) the value of each is equivalent to as many _micra_ as there are millimetres in the focal length of the objective employed. _Wright's Eikonometer_ is really a modification of the eyepiece micrometer for rapidly measuring microscopical objects by direct inspection, having previously determined the magnifying power of the particular optical combination employed. It is a small piece of apparatus resembling an eyepiece, with a sliding eye lens, which can be accurately focussed on a micrometer scale fixed within the instrument. When placed over the microscope ocular the divisions of this scale measure the actual size of the virtual image in millimetres. In order to use this instrument for direct measurement, it is first necessary to determine the magnifying power of each combination of ocular, tube length and objective. Place a stage micrometer divided into hundredths of a millimetre on the microscope stage and focus accurately.