770 



Popular Science Monthhj 



Fig. 7. Apart of the 

 slide-carrier holder 



bottom of the pot and into the brass 

 bracket. The drawing shows how the 

 disk of clear glass is clamped to the 

 front of the flower pot. The method of 

 constructing the 

 lamp-mounting, 

 wiiich slides over 

 the brass tubes of 

 the instrument 

 frame, is clearly 

 indicated in Fig. 

 14. The addition 

 of the rear sup- 

 port, detailed in 

 Fig. 15, completes 

 the stereopticon. 

 For the sake of 

 completeness, the details of a small 

 rheostat are given in Fig. 16. While a 

 lamp-bank, or any other form of re- 

 sistance that will limit the flow of cur- 

 rent to about 5 amperes, may be used, 

 the wire resis- 

 tance illustrated 

 will be found 

 inexpensive and 

 compact. The 

 base is a piece of 

 slate about 8 in. 

 square in which 

 are mounted nine 

 brass pillars. On 

 cither side and 

 between these pil- 

 lars are stretched 



9 coils of No. 22 gage resistance wire, 

 each coil containing 5 fl- of the wire. If 

 wound in coils of 48 tu.-s each on a rod 



Fig. 8. Another part 

 of slide-carrier holder 



m. 



*/2 brass w/rs. 



approximately correct. As the drawing 

 shows, the coils arc all placed in series. 



In Fig. 17 is given the diagram of 

 connections. The wires leading from 

 the nearest lamji- 

 socket connect 

 with a switch and 

 fuse. One wire 

 goes directly to 

 the upper carbon 

 of the lamp, and 

 the other wire 

 goes to one ter- 

 minal of the 

 rheostat. l'"roin 

 Fig. 10. Brass wire ,1,,. ^iiond ter- 



rings used to j , j ^ ,^ ^. 



strength cnthc 

 numerous joints rheostat, a WMC 



connects with the lower carbon of the 

 lamp. If the current is direct, the upper 

 or horizontal carbon should be the 

 positive. This can be insured by a 

 simple test; turn on the current at the 

 switch and bring the carbons together 

 for an instant, separating them im- 

 mediately for ' is in. If the current is 

 direct, the arc will cpiickly settle down 

 to an absolutely noiseless burning; if 



-5"- 



Fig. 9. Dimensions of the uppyer and lower 

 frame parts of the slide-carrier holder 



alternating, the arc will emit a low hum. 

 If direct current is used, turn the 

 current ofif after the arc has burned a 

 few moments and immediately observe 

 the carbons; the positive will be the 

 hotter one and if this should, by chance, 

 be the lower, the wires at the lamp 

 terminals should be transposed so as to 

 establish the positive pole at the upper 

 carbon. With a direct current arc, 

 practically all (jf the light comes from 

 the positive carbon, while with an 

 alternating current, both carbons give 

 off the same amount of light. 



When the light has been tested, the 

 stereopticon may be set up for use. 

 Place the instrument upon a firm table 

 and at right angles to the screen, which 



iV 



in diameter, the a.nount will be Z- 



•oh.^ 





4 reg. brass 



•^l\j 





4 rcg. brass 



4 rcg. brass 

 Fig. 11. Dimensions of the collars 

 which arc forced over the tubes and sol- 

 dered in place gripping the lens-board 



may be a slieet or a bare white wall. 

 Start the light at the arc and darken the 

 room. I'lace a slide in the carrier and 

 bring the lens into focus by sliding the 



