Outerbridge.] 17^ [May 15, 1874. 



DESCRIPTION OF FIGURES 1 AND 2.* 



A is a cast-iron base supporting the brass stem C which has a thread 

 cut upon its lower end in order tliat it may be raised or lowei'ed in the 

 base A and firmly held in position by the jam nut B. 



Into the stem C, a secondary stem D is screwed ; this may be raised 

 or lowered in stem C by turning the hand-wheel GH, 



Upon the upper end of the secondary stem D is fitted a cylinder com- 

 posed of the metallic band FF and the insulating centre G. This cylin- 

 der is held in position by the collars on either side, and is kept from ro- 

 tating by a pin passing through the upper collar and sliding in a slot in 

 the third stem E. 



Through the stem C and secondary stem D passes a third stem E of 

 steel, having upon its upper end a cylinder similar to the one before de- 

 scribed, except that it is slotted to receive twelve strips of metal, while 

 the lower cylinder is slotted to receive one strip. This cylinder is free to 

 turn upon the stem E, and is fixed at any point by the nut upon the end 

 of the stem. 



The stem E is prevented from turning by a pin sliding in a slot in the 

 lower stem C. 



The pitch of the screw upon the stem E, is twice that of the screw on 

 the lower end of the secondary stem D. In turning the hand- wheel 

 GH in either direction, the stem E with the upper cylinder, though 

 moving over twice the distance of the lower cylinder, yet moves an equal 

 distance from a central point between the two cylinders, because the lower 

 cylinder in moving from the central point carries with it the upper cylin- 

 der. It is to overcome the distance lost that the pitch of the screw upon 

 the stem E is doubled. 



The lower portion of the secondary stem D is divided into 24 degrees. 

 A movement of a degi-ee separates the electrodes ais of an inch. 



*NoTE — Fig. 2 is reproduced by Mr. Carbutt. of Philadelphia, from 

 the original drawings according to a modification of the Woodbury 

 Photo- relief process. 



With Fig. 2 are given in this No. of the Proceedings, and by the same 

 process, fac-similes of two sets of drawings of spectra of various alloys 

 described in the above memoir. 



