( 692 ) 
eftabliflbiDg the Longitudes overall the Earth. For^befides that 
thefe Eelipfes are very frequent , the Emerfiod and Iitimerfion 
of thefe Satellites^t^ptcizWy in the fliadow of "jufiter , is fo 
momentany and fo fenfiblc, that they may be obferved with the 
greateft exaftnefs, being altogether exempt from thofe effential 
inconveniencies that accompany the Eelipfes of the Sun and 
Moon, which alfoare rare, and whofe beginning and end are al- 
waies doubtful by reafon of a certain ambiguous light. 
The Longitudes of places at Sea, Capes, Promontories, dind 
divers Iflands being once exafily known by this means, would 
doubtlefsbeof great help and conGderable ufefulnefs to Na- 
vigation. 
Since Monfieur Borelli hath found this way of working 
Glaflcs y he entrufted the fecret of it to a perfon of the Aca- 
demy above-mentioned 5 and he purpofeth to publifli the fame 
hereafter,with fome other confiderable Obfervations touchipg 
the fame Glaflcs. 
ji Letter from Liege concerning Mr. Newton / Experiment 
of the colour edSptfkruwi'-i together with fome Exceftions 
againji. his Tbeorf of Light and Colours, 
Hon^'^Sir, 
MR Gafioigne having received your obliging Letter of 
Jan.i %y with frefh diredions from Mr, Jxey0tm\ hxnt 
wanting convenience to make the.Expfriment according to the 
ftidiriffruaions^ he has requefted me . to fupply his want. In 
compliance with his requeft I have made many Trials ; the 
ilTue whereof I here acquaint you with: next, with fome ex- 
ceptions, grounded on Experiments, againft-j^r,i\r<^^^ new 
Theory of Light Colours. \ i'^i 
The vertical angle of my Prifm. was 6a«^|;;thcdiftance of the 
Wall, whereon the coloured ^p^^r«A»appeared,frdm the Win- 
dow,about 18 foot : The diameter of ihe Hple inthe Window* 
fliuts in length the liqeif^whic^ UpcpA ocf I con- 
trafl'ed to half the faid diameter 5 bul^jftill with eqiial 
fuccefs as to the main of the Experiment.The refraAi- 
ons on both fides the Frifm, were as near as I could make them, 
equal. 
