The pressure at which the maximum heating of the anode by the cathode rays occurs is a good deal higher than that at which the maximum Roentgen effect is produced. There is little doubt that the Roentgen radiation changes in nature to some extent as the vacuum improves either as a primary or secondary effect. It is therefore of some importance to test the tube for the purpose for which it is to be used during the actual exhaustion. It has been stated, for instance, that the relative penetrability of bone and flesh to Roentgen radiation attains a maximum difference at a certain pressure; this is very likely the case. Whether this effect is a direct function of the density of the gas in the tube, or whether it is dependent on the voltage or time integral of the current during the discharge, are questions which still await a solution. The preparation of calcium tungstate for fluorescent screens is very simple. Commercial sodium tungstate is fused with dried calcium chloride in the proportion of three parts of the former to two parts of the latter, both constituents being in fine powder and well mixed together. The fusion is conducted in a Fletcher's crucible furnace in a clay crucible. The temperature is raised as rapidly as possible to the highest point which the furnace will attain--i.e. a pure white heat. At this temperature the mixture of salts becomes partly fluid, or at least pasty, and the temperature may be kept at its highest point for, say, a quarter of an hour. At the end of this time the mass is poured and scraped on to a brick, and when cold is broken up and boiled with a large excess of water to dissolve out all soluble matter. The insoluble part, which consists of a gray shining powder, is washed several times with hot water, and is finally dried on filter paper in a water oven. In order to prepare a screen the powder is ground slightly with very dilute shellac varnish, and is then floated over a glass plate so as to get an even covering. Unless the covering be very even the screen is useless, and no pains should be spared to secure evenness. It is not exactly easy to get a regular coat of the fluorescent material, but it may be done with a little care. CHAPTER II GLASS-GRINDING AND OPTICIANS' WORK Sec. 52. As no instructions of any practical value in this art have, so far as I know, appeared in any book in English, though a great deal of valuable information has been given in the English Mechanic and elsewhere, I shall deal with the matter sufficiently fully for all practical purposes. On the other hand, I do not propose to treat of all the methods which have been proposed, but only those requisite for the production of the results claimed. The student is requested to read through the chapter before commencing any particular operation.