EXAMINING CONTACT.
abrasives employed for polishing are various earths and metallic oxides, such as tripoli, rouge, and putty powder (oxide of tin), and these are applied with water upon rubbers made of some soft sub stance. Cheap lenses are frequently polished with rubbers made of paper or cloth, which act quickly but are apt to destroy the perfect sphericity of the lens by over-rubbing and rounding the outer margins. For polishing the best lenses, rubbers made of waxes or resins are used ; these, although so soft as not to scratch the glass, are inelastic, and cannot, if properly applied, distort the curvature of the lens. These wax polishers are held in supporting shells of metal. Exceptional skill is required for this work, and much of it has to be done by hand to secure the best result. In intervals during the process the work and the tool are cleaned, and fresh abras ive applied to continue the operation until it is complete. At each interval the lens is carefully examined, and as it ap proaches completion the accurate forma tion of its surface is tested by means of what is called a contact gauge. This con
sists of a piece of very hard glass (Fig. 536), having en it a spherical surface, ground and polished with great accuracy, an exact counterpart of the surface it is desired to test. When the gauge and the lens to be tested are very carefully wiped, placed together, and viewed by reflected light, as shown in Fig. 537, brilliant colours are seen, formed -by interference at the two contact surfaces, colours which exactly resemble those seen in soap bubbles. These colours can only be pro duced when the two surfaces which form them arc exceedingly close together. They have their maximum brilliancy when the surfaces are a few millionths of an inch apart ; and if the separation of the surfaces varies, the colours change and spectrum bands are formed. It is by noting the brilliance, the form, and time separation of these bands of colour that the lens maker is able to measure and to work with so great a degree of precision and exactness.