SCIOPTICON MANUAL. 133 



motions there is an advancement of the wave, while the 

 individual particles only rise and fall without advancing. 



The slips of glass, mentioned above, can be con- 

 veniently prepared for drawing diagrams, by coating one 

 side with plain collodion (gun cotton dissolved in equal 

 parts of alcohol and ether) ; when dry this surface takes 

 India-ink admirably, and diagrams can be traced, or 

 pictures copied in a rough way, by laying the glass plate 

 so prepared over the picture to be copied and tracing 

 its outline with a pen filled with good India-ink. 



I would strongly advise any one using your lantern to 

 procure some of the comic elides, such as you illustrate 

 in Class XY of your catalogue of slides, and they can see 

 how to make similar ones to be used in illustrations 

 of scientific subjects. Thus with the wreck of one of 

 these three glass slides, picked up at some opticians and 

 purchased for a few cents, I improvised a slide which 

 answered better to illustrate the process of carbon print- 

 ing in photography than the process itself would have 

 done in a lecture-room. One figure changed with 

 another by means of sliding glass plates is very useful 

 in many kinds of experiments or illustrations of facts 

 and processes. 



The tank figured in your manual, in Chapter YII, on 

 Chemical Experiments, contributed by Prof. Morton, 

 can be made to do service in a long line of experiments 

 with electricity, by a very simple device. Thus, to illus- 

 trate the decomposition of water, cut a slip of segar-box 

 wood, of a size that will lay on the bottom of the tank 

 loosely, attach to this bit of wood copper wires, which 

 Will extend up to the end of the tank and will not quite 

 meet at the centre of the bit of wood; to upturned ends 

 at this place, solder little slips of platinafoil, f inch long 

 by i inch wide, they must stand vertically face to face, 



