VOL. LXXXVII.] PHILOSOPHICAL TRANSACTIONS. 211 



I then applied the other edge so near that their spheres of flexion might interfere 



in the manner before described *, and thus the fringes might be distended : still 



no uncommon appearance took place ; nor when other bodies were used with one 



i . „r ^-jrofal nor when polished pieces of different shapes and sizes were em- 

 ployed. The same things happened uy uuu.,-,> & «.., _.. ^ . , , w _* .. ..., Urktnnm ^ 



neal light. In short, I repeated most of my experiments on flexion with Iceland 



crystal, and found that they were not changed at all in their results. 



Observ. 4 . Having great reason to doubt the accuracy of an experiment tried 

 by Mr. Martin, and in which, by a prism of Iceland crystal, he thought 6 spectra 

 were produced, I was not much surprized to find, that a prism made by polishing 

 the 2 contiguous sides of a parallelopiped of Iceland crystal produced only 2 equal 

 and parallel images, in whatever position the prism was held. But though, from 

 the imperfect account which Martin gives of this appearance, it was impossible to 

 discover his error from his own words, yet chance led me to find out what most 

 probably had misled him ; for looking at a candle through the opposite sides of a 

 specimen of Iceland crystal, I saw 4 coloured images, besides 2 white ones, of the 

 candle. These were parallel to one another, and in the same line, as represented 

 in fig. 11, where e represents the two regular images, g and p two others coloured 

 very irregularly, and changing colours as the crystal was moved horizontally, some- 

 times appearing each two-fold, and its 2 parts of the same or different colours. 

 A and b were regularly coloured, and evidently formed by refraction, and reflected 

 back from the sides. On turning the crystal round, so that its position might be 

 at right angles to its former position, the images moved round, and were in a line 

 perpendicular to ab, as cd. All this happened in like manner in the sun's rays ; 

 and on viewing the specimen, I found it was split and broken in the inside, so as 

 to be lamellated in directions parallel, or nearly so, to the sides ; on these plates 

 there were colours in the day-time by the light of the clouds : and it is evident that 

 it was these fractures which caused the irregular images g and p, for other speci- 

 mens showed no such appearance. I would therefore conclude, that Iceland crys- 

 tal separates the rays of light into 2 equal and similar beams by refraction, and 

 no more *j~. 



As to the cause of the separation, I would hope that some information may be 

 obtained from the experiments I have related : for from them it appears, that this 

 singular property extends no farther than to the action of the particles of Iceland 



* Phil. Trans, for 1796, page 256. — Orig. 



+ Mentioning this account of Martin's mistake to Professor Robison, of this university, I was pleased 

 to find a full confirmation of it. It was that excellent philosopher who showed die appearance to Mar- 

 tin j but he not understanding it, took the liberty of publishing the observation as his own, after first 

 mangling it in such a way as to give him indeed some pretext for the appropriation. The Professor 

 merely mentioned his having communicated it to Mr. Martin ; how the latter used it we have shown in 

 the text : the theory of the appearance is somewhat more complex than appears by my observations. I 

 was therefore pleased to find that the Professor was in possession of the true account of it ; which is 

 however foreign to the present purpose. — Orig. 



£ £ 2 



