^92 Discoveries of Erasmus Bartholinus^ 
pear to be a copy of this work in this country, and we have 
learned that it is not to be found in the library of the Royal 
Society ; but the want of it is well supplied by ‘‘ An aecount of 
sundry experiments made and communicated by Dr Erasmus 
Bartholin, upon a crystal-like body sent to him out of Ice- 
land,” addressed to Dr Oldenburgh, and printed in the 67th 
Number of the Philosophical Transactions. From this account 
we shall select those parts which relate to double refraction, and 
shall in general give them in the words of the author. 
‘‘ 1. The objects seen through” this crystalline prism appear 
sometimes, and in certain positions of the prism, double ; where 
’tis to be noted, that the distance between the two images is 
greater or less, according to the different bigness of the prism ; 
insomuch, that in thinner pieces this difference of the double 
image almost vanisheth. 
% The object appearing double, both images appear with a 
fainter colour ; and sometimes one part of the same species is 
obscurer than the other. 
3. To an attentive eye, one of these images will appear 
higher than the other. 
“4. In a certain position, the image of an object seen through 
this body appears but single, like as through any other tran- 
sparent body. 
5. We have also found a position wherein the object ap-i 
pears sixfold 
6. If any of the obtuse angles of this prism be divided into 
two equal parts by a line, and the visual rays do pass from the 
eye to the object through that line or its parallel, both images 
will meet in that line, or in another parallel to it. 
7. Whereas objects seen through diaphanous bodies, are wont 
to remain constantly in the same place, in what manner soever 
the transparent body be moved, nor the image on the surface 
move, except the object be moved; we have observed here, 
that one of the images is moveable, the other remaining fixed ; 
although there be a way also to make the fixed image moveable, 
* The sextuple multiplication of the image here mentioned, arises, as will be 
seen in a subsequent article, from thin contemporaneous veins of Iceland spar in^ 
tersecting the rhomboidal mass. See Phil. Trans, 1816, p. 270; and Edin. Trans^^ 
yol. viii. p, 165. 
