1 63 'Mr. herschel’s Defer Ip i ton of 
>from o to 90* by ufing the handle P, and alfo to any diftarice 
from fix-tenths of an inch to five or fix and twenty inches by 
means of the handle D. If any reflection or appearance of 
lio ht fhould be left from the top or fides of the lamps, a tempo- 
irary fereen, con lifting of a long piece of pafte-board, or a 
wire frame covered with’ black cloth, of the length of the 
whole arm and of any required breadth, with a flit of half an 
inch broad in the middle, may be affixed to the arm by four 
bent wires projecting an inch or two before the lamps, fituated 
fo that the moveable lucid point may pafs along the opening 
left for that purpofe. 
Fig. 2. reprefents part of the arm L, half the real fize ; S 
the Aider ; m the pulley, over which the cord xtyz is returned 
towards the center ; v the other cord going to the pulley n of 
fig. 1. R the brafs piece moveable upon the pin c, to keep the 
lamp upright. At R is a wire rivetted to the brafs piece, upon 
which is held the lamp by a nut and ferew. Fig. 3. 4. repre- 
fent the lamps a, k, with the Hiding doors open, to fhew 
the fituatioii of the wicks. W is the leaden weight with a 
hole d in it, through which the wire R of fig. 2. is to be pafted 
when the lamp is to be faftened to the Aider S. Fig. 5. repre- 
fents the lamp a with the Aiding door (hut ; / the lucid point ; 
and ik the openings at the top, and s at the tides for the ad- 
miffion of air. 
Every ingenious artift will foon perceive that the motions of 
this micrometer are capable of great improvement by the appli- 
cation of wheels and pinions, and other well known mecha- 
nical refources ; but, as the principal objeCt is only to be able 
to adjuft the two lucid points to the required poAtion and dis- 
tance, and to keep them there for a few minutes, while the 
: * . . * * - obferver 
