1^8 On the Proptrtles of Light.— IcehrJ Ci^Jlal 



round the image. Such at leaft I take to be the explanation of the phenomenon obferyei 

 at Paris by M. de Barros during the tranfit of mercury in I743> and recorded in Phil. 

 "Iranf. for 1753. But there is another more ferious impediment to the performance of re- 

 ^leiSlors, and which it is to be feared we have no means of removing. In making the experi- 

 ments of which the hiftory has been given, on viewing attentively the furface of the fpecu- 

 lum, every part of it was feen covered with points of colours, formed by reflexion from the 

 fmall fpecular particles of the body. I never law a fpcculum free in the lead: from thefe 5 fo 

 that the image formed in the focus mull: be rendered much more dim and confufed by thein 

 than it otherwife would be. 



Ill, The laft conclufion which may be drawn from thefe experiments is a very clear de- 

 •monftration in confirmation of what was otherwife fhewn, concerning the difference be- 

 tween coloured images produced by reflexion and thofe made by flexion. This complete 

 diverfity is moft evident in the experiments with fpecula, the colours produced by which, 

 ■in the form of fringes and rings, ought, as well as the others defcribed as images by reflexion 

 in Obf. 1 1, to be the fame in appearance with thofe formed by pins ; whereas no two things 

 can be more diflimilar. 



It remains to examine the 6th propofition. For this purpofe I made the following 

 obfervations : 



Obfervation i. Having procured a good fpecimen of Iceland cryftal, I fplit It into feveral 

 pieces, and chofe one whofe furface was beft polifhed. I expofed this to a fmall cone of the 

 ■fun's light, and received the refleded rays on a chart; nothing was obfervable in the image 

 farther than what happens in reflexion from any other poliftied body. Some pieces indeed 

 doubled and tripled the image, but only fuch as were rough on the furface, and confequently 

 prefented feveral furfaces to the rays. When fmooth and well poliflied, a fingle image was 

 all that they formed. The fame happened if I viewed a candle, the letters of a book, &c. by 

 reflexion from the Iceland cryftal. 



Obfervation 2. I ground a fmall piece of Iceland cryftal round at the edge, and gave it a 

 tolerable polifli here and there by rubbing it on looking-glafs, and fometimes by a burniftier 

 (it would have been next to impofiible to polifti it completely). I then placed the poliflied 

 part in the rays near the hole in the window-fhut, and faw the chart illuminated with a great 

 variety of colours by reflexion, irregularly fcattered as defcribed above*. I therefore held 

 the edge in the fmoke of a candle and blackened it all over, then rubbed off a very little of 

 the foot, and expofed it again in the rays. I now got a pretty good ftrcak of images by re- 

 flexion, in no refpeft differing from thofe made in the common way. Nor could I ever 

 produce a double fet or a fingle fet of double images by any fpecimen properly prepared, 

 «ither on a chart by the rays of the fun, or on my eye by thofe of a candle. 



Obfervation 3. I ground to an even and pretty Iharp edge two pieces of Iceland cryftal,j 

 and placed one in the fun's rays. At fome feet diftance I viewed the fringes with which its 

 fliadow was furrounded, and faw the ufual number in the ufual order. I then applied the 

 other edge fo near that their fpheres of flexion might interfere in the manner before de- 

 fcribedf, and thus the fringes might be diftended : ftill no uncommon appearance took 

 place, nor when other bodies were ufed with one edge of cryftal, nor when poliflied pieces 



• Phil. Tranf. for 1796, p. 470, or Phil. Journal I. ,594. f Ibid. p. 256, or Phil. Journal I. 5J7. 



of 



