After being smooth ground the lens is beautifully smooth and velvety to the touch but is just as much ground glass as ever, that is, it is absolutely opaque. We now come to the polishing. This is done with specially prepared rouge and only an excessively small amount of glass is taken off. Lord Rayleigh in a paper on “Polishing of Glass Surfaces” read before the British Optical Convention held in 1905, states: “I started with a finely ground surface, rather more finely ground I think than is used in practice, and I found that in order to obtain a pretty good polish it was necessary to remove a weight of glass, corresponding to a depth of about 6 wavelengths. I do not pretend that such a polish would satisfy the requirements of commerce; probably the 6 would have to be raised to 10 or 12 in order to get to the bottom of the deepest pits.” When it is remembered that a wave length is about the fifty thousandth part of an inch we realize how very delicate such lapping must be. For this work the lap is covered with pitch which has been brought to the proper degree of hardness either by boiling, to harden it or by adding asfalt varnish to soften it. The proper degree of hardness is very important and must be adjusted to the temperature of the room. Obviously if the pitch is too soft it will not hold its shape and it will be impossible to hold the polishing tool to the proper radius. I have put three different curves on a lens about an inch in diameter in a few minutes and it had to go back on the grinding machine before it could be finished.