210 PHILOSOPHICAL TRANSACTIONS. [ANNO J 797- 



the image. Such at least I take to be the explanation of the phenomenon, observed 

 at Paris by M. de Barros during the transit of Mercury in 1743, and recorded in 

 Phil. Trans, for 1753. But there is another more serious impediment to the per- 

 formance of reflectors, and which it is to be feared we have no means of removing. 

 In making the experiments of which the history has been given, on viewing atten- 

 tively the surface of the speculum, every part of it was seen covered with points of 

 colours, formed by reflexion from the small particles of the body. I never saw a 

 speculum free in the least from these, so that the image formed in the focus must 

 be rendered much more dim and confused by them, than it otherwise would be. 



III. The last conclusion which may be drawn from these experiments, is a very 

 clear demonstration in confirmation of what was otherwise shown, concerning the 

 difference between coloured images produced by reflexion, and those made by 

 flexion. This complete diversity is most evident in the experiments with specula, 

 the colours produced by which, in the form of fringes and rings, ought, as well as 

 the others described as images by reflexion in obs. 11, to be the same in appear- 

 ance with those formed by pins ; whereas no 2 things can be more dissimilar. It 

 remains to examine the 6th proposition : for this purpose I made the following 

 observations. 



Observ. 1. Having procured a good specimen of Iceland crystal, I split it into 

 several pieces, and chose one whose surface was best polished. I exposed this to a 

 small cone of the sun's light, and received the reflected rays on a chart : nothing 

 was observable in the image, more than what happens in reflexion from any other 

 polished body. Some pieces indeed doubled and tripled the image, but only such 

 as were rough on the surface, and consequently presented several surfaces to the 

 rays : when smooth and well polished, a single image was all that they formed. 

 The same happened when I viewed a candle, the letters of a book, &c. by reflexion 

 from the Iceland crystal. 



Observ. 2. I ground a small piece of Iceland crystal round at the edge, and gave 

 it a tolerable polish here and there by rubbing it on looking-glass, and sometimes 

 by a burnisher (it would have been next to impossible to polish it completely). I 

 then placed the polished part in the rays near the hole in the window-shutter, and 

 saw the chart illuminated with a great variety of colours by reflexion, irregularly 

 scattered, as described above * ; I therefore held the edge in the smoke of a candle 

 and blackened it all over, then rubbed off a very little of the soot, and exposed it 

 again in the rays. I now got a pretty good streak of images by reflexion, in no 

 respect differing from those made in the common way. Nor could I ever produce 

 a double set, or a single set of double images, by any specimen properly prepared, 

 either on a chart by the rays of the sun, or on my eye by those of a candle. 



Observ. 3. I ground to an even and pretty sharp edge 2 pieces of Iceland crystal, 

 and placed one in the sun's rays. At some feet distance I viewed the fringes with 

 which its shadow was surrounded, and saw the usual number in the usual order. 



• Phil. Trans, for 1796.— Orig. 



