556 Oh the Irjtcx'ion, RcJlixLn, end Cslturs sf Light, 



OIJ. II. Hdving produced a clear fct of colours, as in the laR Obfervation, I viewed them 

 as attentively as poflible, and found that they were divided into fcts, fometimes feparatcd 

 by a gleam of white light, fometimes by a line of fliadow, and fometimes contiguous, or 

 even running a little into one another. They were fpeftra or images of the fun, for they 

 varied with the luminous body by whofe rays they were formed, and with the fii:e of the 

 beam in which the pin was held ; and when by placing it betv/cen my eye and the candle 

 a little to one fide I let the colours fall on my retina, I plainly faw that they refcmbled 

 the candle in fhape and fize (though a little dillcnded), and alfo in motion, (ince, if the 

 flame was blown upon, they had the like agitation, The colours, therefore, wliich fell ou 

 the chart were images of the fun : they had parallel fides pretty diflin£>ly defined, but the 

 ends were confufed and fcmicircular, like thofe of the prifmatic fpcflrum. Like it, too, 

 they were oblong, and in fome the length exceeded the breadth fix, even eight, times. The 

 breadth was, as I found by meafuremcnt, exa£lly equal to that of the fun's image received 

 on a chart as far from the pin as the image was, and the length was always to the breadth 

 at all diftanccs in the fame ratio, but not in all pofitions of the pin ; for, if it was moved on 

 its axis, the images moved towards the fliadow on one fide, and from it on the other, be- 

 coming longer and longer (the breadth remaining the fame) the nearer they came to the 

 ihadow on the one fide, and (horter in the fame proportion the farther they went from it 

 on the other. 



Ohf. 111. Having picked out an image tliat appeared very bright and well defined, I lei it 

 through a hole with moveable fides in the upper part of a fort of deflc, which moved to any 

 opening by hinges, and had a chart for Its under fide, on which the image fell, and I fhut 

 the hole fo clofe as to prevent any of the others from coming through. I then had a fuH 

 opportunity of examining it in all refpcfls, and I counted in it diftin£lly the feven prifma- 

 tic colours ; the red was fartheft from the fhadow of the pin, and from the pin itfelf ; 

 then the orange ; then the yellow, green, blue, and indigo ; and the violet neareft of all : 

 in fliort, it was exaflly fimilar to a prifmatic fpe£lrum much diminifhed in length and 

 breadth, and turned horizontally on the wall oppofite to the prifm, with the red fartheft 

 away. In figure 5, j «• is the pin reflefting the rays C P and C O, which pafs through P O 

 the hole in the delk E D, to the chart or bottom of the defk R T S D ; and from there the 

 fpedlrum I K divided into its colours, I being violet, and K red. On moving the hole in the 

 delk, and letting through other im»ges, the colours were not in all arranged the fame way : 

 but I moved the pin on its axis, and obferved thofe where the order was inverted, to move, 

 not only with refpefl to the pin, but alfo with refpeiS to the contiguous images ; and I was 

 furprifed to fee them aflume the order of colours fird mentioned, namely the red outer- 

 mod, and the violet innermoft. In like manner, the images which before the motion 

 were regular, on moving into the places left by the others had always the order of their 

 colours inverted, fo that the thing muft be owing to fome irregularities in the pin's fur- 

 face i for thofe which were made by a fmall glafs tube filled with quickfilver, and freed 

 from fcratches by a blow-pipe, prefeived during the motion the proper order of colours. 

 Another irregularity in the arrangement was alfo obfervable even in the glafs tube ; for two 

 contiguous images, by mixing one with another for two or three fucceflJons, appeared each 

 to have outermoft a dull colour between red and violet, and inncrmod a green : but here 



unlefi 



