( ) 
tlieother, tttuft be mearared. For in my fir ft Lett^ atetft Co- 
lours, where I fet down the length to be five times the breadth, 
I called that length the uimoft length of the image 5 and I 
Ineafured the utmoft length , becaufe I account all chat length 
to be caufed by the immediate light of the Sun, feeing the Co- 
tours (as I noted above ) become vifible to the grcateft length 
in the cleareft days, that is, when the light of the Sun tranf- 
cends moft the light of the Clouds. Sometimes there will 
happen to flioot out from both ends of the Image a glaring 
light a good way beyond tbefe colours , but this is not to be 
regarded, as not appertaining to the Image. If the meafures 
he taken right, the whole length will exceed the length of the 
ftreight fides by about the breadth of the Image. 
By thefe things fet down thus circumftancially, I prefume 
Mr. will be enabled to accord his <ryals of the Experi- 
ment with mine ; fonearly, at leaft, that there (hall not remain 
any very confiderablc difference between us. For, if fomc lit- 
tle difference fliould ftill remain ^ that need not trouble us any 
further, feeing there may be many various circumftances which 
may conduce to it 3 fuchas are not only the different figures 
of prifms, butalfo the different refradive power of GlafTes, 
the different diameters of the Sun at divers times of the year, 
and the little error's that may happen in meafufing lines and an- 
gles , or m placit^ the ptifm at the window 5 Itliough, for my 
part^ I took care to do thefe things as ^xa61y as I could. How- 
ever Mr. Lticas may make fare to find the Image as long or lon- 
ger than I have fet down , if he take a priftn whofe fides afetioc 
hollow ground, but plain, 6r (which is better) a very littfe 
convex, and whofe refrafting angle is as much greatef than that 
I ufed, as that he has hitherto rryed it wltS', is lefs 5 that is , 
whofe angle is about 66 or 67 degrees, or (if he will) a little 
igreatef. 
Concerning Mr. Lucas's other Experiments , I am much ob- 
liged to him that he would take thefe things lb far into confide- 
ration, and be at fo much pains for examining rhem j and I 
"'thank him fo much the more ^ becaufe he is the firft that has 
fent mean experimental examination of them. By this I may 
prefume he really defires ro know whit tiruth there i^ W thefe 
matters. But yet it will conduce to his mtt Pp^ddf artd full 
fatif. 
