( 2^7 ) 
apparent, by feeing On one of thefe Images the Reprc- 
fentation of the black Threads, that are wrapp’d round 
the Card, whilft they are not vifible on the other. 
This is fully defcribed in the Place abovemention’d ; 
but yet a * Gentleman abroad has call’d the Experiment 
in queftion,and denied the matter of Fadi, faying, that 
he could nor make it liicceed, but propofes an Expe- 
riment ofhis own to difprove the different Rcffangibi- 
lity of the Rays. 
Upon this I was defir’d to make the Experiment 
over again, before the Royal Society, which fucceeded 
well. But becaufe there muft be care taken in the 
making it, I fhall mention all the Particulars obferved in 
the Performance ; which, if duly put in pradiice, will 
make the Experiment always fiicceed. 
I painted one half of the Card RB, {Fig, X.) as B, 
with Ultramarine^ made deeper with a fhaall Mixture 
of Indigo^ and the other half R, I painted over with 
Cinnabar heighten’d with a little Carmine^ fb that the 
Line, that feparated the red from the blue, was per- 
pendicular to the long fides of the Cord. 
Then I wrapp’d a black Silk four times together, 
over the middle of each painted part of the Card, as in 
Fig. XL 
Upon a fquare Trencher, Fig. XII. painted black,, 
and liifpended vertically againfl a Wall, I fix'd my co- 
lour’d Card with a Pin, and the Room being made very 
dark, I enlighten’d the Card with a ftrong Light thrown 
upon it from a dark Lanthorn, that had two Convex 
Glafles in it ; then letting up the Lens L L, (repre- 
lented by Fig. XIII.) in fuch manner, that its Axis 
pafs’d perpendicularly thro’ the Image of the Card, at 
* A£t. Erudit. Lipf. Supplem. tom. 8. 3. p. 130. 13 1. 
L I X 
the 
