148 On the JfeBkns and Properties of Light. 



caufed the images to be of all the colours. When the prifm was turned round on its axis, 

 fo that different rays fell on the pin, the images changed their fizes as well as their po- 

 fitions : they were largeft. when red, and leaft when violet. 



Obfervntion 3. — In cafe it may be thought that the fides of the hole, through which the 

 rays pafled in Obfervation 2, by inflecSling, might difpofe them, before incidence, into 

 beams of diflerent fizes, I removed the fcreen, and placed the pin horizontally, the axis o£ 

 the fhadow being now at right angles to that of the prifmatic fpedlrum ; and moving the 

 prifm on its axis, again I obferved the contraclion, and dilatation of the images by re- 

 fle£lion, though now they were rather lefs dillinct, from the greater fize of the incident 

 beam ; and to fliew that there was both a change of fize and of place, without any manner 

 of deception, I placed one leg of a pair of compalTes in a fixed point of the fpe£i:rum, and 

 the other in the middle point of an image formed by the violet-making rays. The prifm 

 being then moved till the image became red, I again bifecled it, and found its' centre 

 confiderably beyond the point of the compaffes, which was indeed evidently much nearer 

 one end of the image than the other; befides that the red image, when meafured, was 

 longer than the reft : and this fatisfied me that there were two changes, one of place, 

 with refpeft to the fixed point, the other of fize, with refpeft to the centre of the 

 image. Laftly, as far as I could judge, the dilatation and contradtion appeared even and 

 uniform. 



Obfervation 4. — I remarked that the fringes or images, by flexion, were always increafed 

 in fize when formed out of red-making rays, and were lefs in every other colour, and leaft 

 in violet (befides being moved farther from the edge of the fhadow in the former rays than 

 in the latter) ; and this agrees with an obfervation of Sir Ifaac Newton, as far as he tried 

 it, whiclv was with refpeft to deflexion. In making feveral experiments with prifms, I hit- 

 on a very remarkable confirmation of this. I obferved on each fide of the fpeftrum four 

 or five diftinft fringes, like the images by reflexion, coloured in the order of the fpeftrum, 

 but quite well defined at the edge, and even pretty diftin£l at the end : they were alfo 

 much narrower than thofe images, but like them they inclined much to the violet, and 

 were broadeft in the red, growing narrower by degrees, and narroweft of all in the violet. 

 I moved the prifm, and they difappeared ; but when the prifm was brought back to itsfor* 

 mer'pofition, they alfo returned. 1 thsn obferved the prifm in open light, and faw that 

 it had veins, chiefly opaque and white, running through it, and that there were feveral of 

 thefe in the place where the light pafled when the prifm was held as before. But in cafe 

 the inclination and (hape of thefe images might be owing to the irregular order in which 

 the veins were laid, I held another prifm, which happened to have parallel veins : in many 

 pofitions of this the fringes or images returned, not indeed always fo regular nor always 

 of the fame kind j for fomc were confufed and broader, formed (as I concluded from this 

 and their pofition) by reflexion ; others made by tranfparent veins and air-bubbles were 

 alfo irregular, but inclined to the red, the violet being fartheft from the perpendicular, 

 and thefe were obvioufly caufed by refraftion ; yet all agreed in this, that they were broadeft 

 in the red, and narroweft in the violet parts. 



Obfrvation 5. — I held, in the direft rays of the fun at half an inch from the fmall hole in 

 the window-fhut, a glafs tube, free from fcratches and opaque veins, but, like moft glafs 

 that is not finely wrought, having its furface of a ftru6lure fomewhat fibrous. When this 



tube 



