The surfaces of good spectacle lenses depart from true spheres by amounts of the order of one ten-thousandth of an inch, while the optical work in good binoculars rarely has errors of as much as one hundred-thousandth. The best optical work departs from the ideal aimed at by less than one millionth of an inch. Gauges for the smaller optical tools may be prepared by turning a disc of brass about two millimetres thick to the requisite diameter as measured by micrometer, the edge being reduced to about one half millimetre. There is a wide range of materials available which have been adopted as standard by one or other of the optical houses whose methods we have been privileged to examine. There are three materials in general use for polishing optical work; these are rouge, cerium oxide and putty powder. The best rouge for lens polishing should have a good red colour and should contain little or no free sulphate.