Oh the Inffxkn, R,J}t'S-hn, am! Colotirs nf Light. ^^ 



t . It lias been remarked, that if we look at a candle, or other lu-minous body, witli our 

 eyes almoft (hut, bright ftreaks feem to dart upwards and downwards from it. NcwtKMi* 

 explains this by refraction through the humours adhering to the eyelids ; Rohauk f an4 

 Mr. Young % afcrlbe them to rellexioWs ; Dcfcartes makes them arife from wrinkles on the 

 eye's furface ; De la Hire from refraftion through the moifture on the eyelids, as through a 

 concave lens ; and Prieftley 1| from inflexion through the lalhes. The truth of Sir Ifaac's 

 Dbfervation is obvious, becaufe the ftreaks which dart from tlie top of the luminous body- 

 are formed by the under eyelid, or at lead by the moifture adhering to the under ciliary 

 procefs, and thofe which appear from the bottom of the body by the upper eyelid ; which 

 could not be, either if they were formed by reflexion from the procefles, or by inflexion 

 through the lafhes. 



I have however obferv-ed another kind of ftreaks, mottled with broken colours of all 

 Icinds, and formed by reflexion from the moifture on the procefles. In thefe the under 

 flreak correfponds to the under procefs, and 'uke •vcrfa. They may be formed by any po- 

 liflied body held in the proper pofition between the pupil and luminous body. The colours 

 are very beautiful when made by the fun, and referable, in form and irregularity of arrange- 

 ment, fome of the ftreaks made by large half-poliftied bodies, as defciibed in Part If. of 

 this paper. 



2. The next objeft of attention is one of the greateft importance to our theory, namely, 

 the formation of images by reflexion. Three things here require explanation : the nunlber 

 of the images, their colours, and their variations in point of fite. 



Ohf. 15. I .have uniformly found that no refleifling furface forms them except it be curve, 

 and (its furface) of a ftrufture fomewhat fibrous. A plain mirror, nor a concave, nor a I 

 convex one, do not make them, unlefs they are of that ftrudlure ; and for the fame reafou 

 «}uickfilver, when held fo as to refleft the light incident upop it, forms them not ; but by tri- 

 turating it, fo as to divide it into fmall particles, and by placing thefe in the beam of the 

 fun's light, each particle formed an image with the colours in the regular order, and very 

 bright. On holding a cyHnder in the rays, and obferving the lengths of the images, I 

 found that if the cun-ature was increafed, the images were alfo increafed in fize, being 

 more diftended and highly coloured. Thefe things immediately fuggeft the explanation. 

 Each of the fm.tlt fibres forms an image, which, from the different reflexibiiity of the rays, 

 IS divided into the feven primary colours. But why docs not a plain mirror form one of 

 thefe upon the fame principles ? In fig. X2. let A E be the curve furface of a very convex 

 mirror, that is of a fmall fibre, GC a ray reflefled by the fmall furface DC. It will be fc- 

 parated into CI red, and CK violet, by the unequal a£lion of FC on its parts. But if DC 

 ie continued to L in a ftraight line, then LC's fphere of reflexion extending a little way be- 

 yond it to KC, the part neareft to C, and not to IC, will drive KC, and alfo the indigo and 

 part of the blue, nearer to the perpendicular ; then IC being within LC's fphere of inflexion, 

 •will, together with the orange, yellow, and part of the green, be brought nearer to KC ; fo that 

 IC and KC will both be brought to an angle equal to that of incidence, and will be reflcfted 

 jn a parallel white beam. If LC is removed a little, or the furface becomes more convex, 



* LcG. Opt. Scft.lII. ad fincm. + Pliyfica, p. 449. Clark's eiliu 



X Phil. Tranf. 1793. || On Vjl'iun, vul- u. 



Vot. I.— March 1798.— Sum.. 4G IC 



