294! Discoveries of Erasmus BartJioUmis, 
the moveable to the eye, Unusual, And hence, namely, from 
this peculiar and notable property of the double refraction in 
this Iceland-stone, we have not scrupled to call it Dis-dlaclasticle, 
This being supposed, it will not be irrational to suspect that 
these two refractions proceed from dilferent principles. For, 
since it is commonly known from dioptricks that an object, by visu- 
al rays affecting the eye, exhibits some image on the superficies of 
the diaphanous body, which image is but one as long as the su- 
jperheies is one, and the upper plain parallel to the lower ; as 
also, that if, the eye remaining steady, the diaphanous body be 
moved, that image remains always fixed, as long as the object 
whence it comes remains unmoved. Wherefore in this transpa- 
rent substance, the image which appears fixed may proceed ac- 
cording to the ordinary laws of usual refraction ; but that which 
moveth, and is carried about according to the motion of the di- 
aphanous body, while the object remains unstirred, sheweth an 
unusual kind of refraction, hitherto unobserved by Dioptricians. 
Hence, that I might examine the nature and difference of 
both, I put upon some object, as the point A, Fig. 1. Plate VII. 
the prism of my double refracting crystal NPRQTBS, and the 
eye M being perpendicularly posited over the upper plain of 
the prism NPRQ, I noted whetlie^ there was any refraction of 
the point A, for the usual laws of refraction teach that there is 
none. But the perpendicular ray of the eye was observed to 
pass not through the moveable but the fixed image, thereby be- 
ing conformable to the rules of usual refraction, as striking the 
eye unrefracted, so that the eye, the image, and the object were 
seen in the same line. But when in the same scite of the eye, 
the object A did also exhibit the other image X, at no small 
distance from the former, I took notice that this object A was 
not seen unrefracted by the means of the image X, though the 
eye M remained perpendicular over the plain ; and that, conse- 
quently, this unusual refraction is not subject to the received 
axiom of dioptricks, wlrich imports, that a ray falling perpen- 
dicularly on the superficies of a diaphanous body, is not refrac- 
ted, but passeth unrefracted. 
Next, I so placed the eye in O, that the ray from the object 
A arriving to the eye, might be parallel to the lines RT and 
QB of the plane RQTB, &c. ; then it appeared, that the rays 
were trajected from the object A without refraction, through 
