( 7^4 ) 
The way therefore that I propound , being the fiiorteft and 
cleareft (not to fay, the only proper way,) J queftion not but 
Ur.Lucas mil be glad that I have recommended it, feeing he 
profeffes, that it is the knowledge of trfitb thsit he feeks after. 
And therefore at prefent I fhall fay nothing in anf^er to his 
Experimental difcourfe, but this in general ; that it has pro- 
ceeded partly from fome mifunderftanding of what he writes 
againft, and partly from want of due caution in trying Expe- 
riments 5 and that amongft his Experiments there is one, which 
when duly tried, is,next to the Experimentrnt Cruets, the moft 
confpicuous Experiment,! know, for proving the different re- 
ffangibility of Light, which he brings it to prove againfl-. 
By the Poft-fcrift oi Mr. Lucas's Letter, one not acquainted 
with what has paffed, might think, that he quotes the Obferva- 
tionof the JR.StJW/^'againftme^ whereas the relation of their 
Obfervation, which you fent to Liege, contained nothingatall 
about the juft proportion of the Length of the Image to its 
Breath according to the angle of the Prifm, nor any thing more 
(fofaras I can perceive by your laft) than what was pertinent 
^0 the things then indifpute,i//^. that they found them fuccced 
as i had affirmed. And therefore fince Mr, L»^4(f has found the 
fame ruccefs, I fuppofe, that when he exprefied , that he much 
rejojiced h fee the TrUls of the R. Society agree fo exaHly with 
his^ be meant only fo far as his agreed with mine. 
Andbecaufe I am again upon chis firfl Experiment, I fiball 
defire, that Mr- Lucas will repeat it with all the exaOnefs and 
caution that may be, regard being had to the information about 
it, fet down inthis Letter ; and then I defire fo have the length 
and breadth of the Image with its dift^ame from the Prifm , fet 
down cxadly in feet and inches, and parts of an^ inch, that I 
may have an opportunity to confider what relation its length 
and breadth have to the Suns diameter. For I know , that Mr. 
L«r^Obfervation cannot hold where the refrafting angle of the 
Prifm is full Go degrees, and the day is clear, and tbefull length 
of theColours is mea(ured,and the breadch of the Image anfwers 
to the Sun's diameter : And feeing I am well affured of the trutk 
and cxadnefs ofmy own Obfervations, I lhall be unwilling to 
be diverted by any other Experiments, from having a fair end 
made of this in the firft place. Sir, J am^&c. 
Poft- 
