376 BE.XJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



this extends beyond twenty feet, it will be necessary to 

 change one of the object-lenses in each tube for those of 

 longer foci; this is easily effected, by drawing out the 

 tube containing the lenses, removing the one loose in the 

 cell, and substituting one of longer focus. Perfect coin- 

 cidence of discs is obtained, laterally, by moving the 

 prism on its hinged joint, and perpendicularly, by mov- 

 ing round the tube containing the shutter. The lenses 

 and prisms should be examined, to see that they are clean 

 and free from dust, and carefully placed in their right 

 position. A very little experience will enable those using 

 the instrument to manage the levers and prisms, and, with 

 facility, to produce all the effects desired. When practi- 

 cable, an opaque screen should be used, by having the 

 lantern in front. The best material for a transparent 

 screen is made from the widest cambric muslin, tightly 

 stretched on a frame, and, previous to being used, washed 

 over by a brush with water, to increase its transparency. 

 To Make the Oxygen Gas. The process is so easy and 

 safe, as to render the use of the prismatic lantern in this 

 respect more convenient than that of the oxy-hydrogen, 

 where large quantities of both oxygen and hydrogen have 

 to be made ; and where the gases are united, considerable 

 danger is incurred. Oxygen gas is not combustible, and 

 can not, therefore, be attended with danger; it has no 

 smell, and is the vital principle of the atmosphere. To 

 make it, it is only necessary first, to see that the retort 

 is clean, or, at least, free from coal, oil, or any combusti- 

 ble substance (after making the gas, a residuum is formed 

 at bottom of retort, this should always be removed previ- 

 ous to making fresh gas) ; secondly, put into the retort 

 (if the gas-bag is empty) eleven ounces of chlorate of po- 

 tass, and two ounces of black oxide of manganese, in pow- 

 der, well mixed together ; lute round the cover with putty 

 or clay ; screw it down tightly, and put it on a common 

 fire ; and connect the purifier (which should be half filled 

 with water) with the retort, as shown in wood-cut. If the 

 fire be tolerably bright, bubbles will soon rise through 

 the water in the purifier ; when they come fast and regu- 

 larly, without intermission, allowing all atmospheric air 

 to be expelled, connect the purifier with the gas-bag by 

 the flexible tube, and in about ten minutes you will have 

 three feet of the purest oxygen gas ; when the bubbles 



