S34 PHILOSOPHICAL TRANSACTIONS. [anNO I676. 



Private persons who are not in a condition to make engines for large glasses, 

 may at least make use of glasses of 14 or 20 feet, which he is willing to send them, 

 to observe the eclipses of Jupiter's satellites, which happen almost daily, and 

 afford so fair a way for establishing the longitudes over all the earth. For, 

 besides that these eclipses are very frequent, the emersion and immersion of these 

 satellites, especially in the shadow of Jupiter, is so momentary and so sensible, 

 that they may be observed with the greatest exactness, being altogether exempt 

 from those essential inconveniencies that accompany the eclipses of the sun and 

 moon, which also are rare, and whose beginning and end are always doubtful, 

 by reason of a certain ambiguous light. 



Since M. Borelli has found this way of working glasses, he entrusted the 

 secret of it to a member of the academy above-mentioned; and he proposes to 

 publish the same hereafter, with some other considerable observations on the 

 same glasses. 



Exceptions against Mr. Newton's Experiments, and Theory of Light and Colours, 

 By Mr. Lucas of Liege. N" 128, p. 692. 



Mr. Gascoigne having received your obliging letter of Jan. 18, with fresh 

 directions from Mr. Newton ; but wanting convenience to make the experiment, 

 he has requested me to supply his want. In compliance with this request I have 

 made many trials ; theissue whereof I here acquaint you with; next, with some 

 exceptions, grounded on experiments, against Mr. Newton's new theory of 

 light and colours. 



The verticle angle of my prism was 60 degrees, the distance of the wall, 

 whereon the coloured spectrum appeared, from the window, about 18 feet. The 

 diameter of the hole in the window-shutters about \ of an inch, which was 

 occasionally contracted to half the said diameter; but still with equal success as 

 to the result of the experiment. The refractions on both sides the prism, were 

 equally as near as I could make them, and consequently about 48° 40', the re- 

 fractive power of glass being computed according to the ratio of the sines 2 to 

 3. The distance of the prism from the hole in the shutter was about 2 inches: 

 the room darkened to that degree as to equal the darkest night, while the hole 

 in the shutter was covered. 



Now as to the issue of my trials; I constantly found the length of the colour- 

 ed image, transverse to the axis of the prism, considerably greater than its 

 breadth, whenever the experiment was made on a clear day; but when a bright 

 cloud was near the sun, I found it sometimes exactly as Mr. Line mentions, viz. 

 broader than long, especially while the prism was placed at a great distance from 

 the hole. Which experiment will not, I conceive, be questioned by Mr. New- 



