on the Affections and. Properties of Light. 381 
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 F, for 
other specimens shewed no such appearance. I would there- 
fore conclude, that Iceland crystal separates the rays of light 
into two equal and similar beams by refraction, and no more.* 
As to the cause of the separation, I would hope that some 
information may be obtained from the experiments I have re- 
lated : for from them it appears, that this singular property ex- 
tends no farther than to the action of the particles of Iceland 
crystal on the particles of light in their passage ' through the 
body ; and from Obs. 4. it is farther evident, that it is not ow- 
ing to the different properties which Sir Isaac Newton con- 
jectures the different sides of rays to have; for if this were the 
cause, when the rays pass between two pieces of crystal, an 
uncommon flexion would take place. Lastly, another fact 
(mis-stated by BARTOLiN-f and Rome' de Lisle) J shews, that 
the unusual refraction takes place within the body, while the 
* 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 phi- 
losopher who shewed the appearance to Martin ; 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 1 
mentioned his having communicated it to Mr. Martin ; how the latter used it we 
have shewn 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. 
■f Experimenta Crystalli, abridged in Phil. Trans. Vol. V. 
X Cristallograpbie, Vol. I. 
3D 2 
