1 98 jMifcellcmea Curio j a , 



was alio a Bladder ty'd below each Joint at 

 and when it was fill'd with Water it was 

 ty'd above it at n \ fo that no Air could 

 come to the Cement, or infinuate it felf 

 through its Pores or SifTures if any happen'd 

 to be left unclos'd. 



It is not (I think) an unneceffary trouble, 

 that in this account of the Apparatus I have 

 mention'd fo many minute Circumftances, 

 for we found it difficult enough to exclude 

 the Air, and almoft impoffible to difco- 

 ver the very little holes through which fo 

 fubtil a Fluid would freely enter and poffefs 

 the Spaces deferted by the fuhfiding Mer- 

 cury. But with all this Precaution the Ex- 

 periment fucceeded at laft, as I wiflfd, af- 

 ter this manner. 



We plac'd the Object a (which was a black 

 Thread Aiding in a little Frame over a piece 

 of white Paper) in the Axis of the Cylinder 

 cx continu'd to it, we filled the Pipes and Cy- 

 linder with Mercury ; and having ftopt the up- 

 perm oft Pipe at F with the little Iron-ftcpple 

 K and clos'd it at the other Joints, we let the 

 Mercury run out gently at O into the Blad- 

 der i>, till it remained fafpended at the ufual 

 height (as in the Barometre) leaving the 

 fpace above it between the Glaffes 00 and ff 

 void of Air. We then found the Objed, 

 which before appear'd in the Axis at x, rais'd 

 confiderably above it } and we reduc'd it to 

 appear at at by removing it from a to x* 

 The Axis therefore, of the vifual Ray xa % 

 (which was alfo the Axis of the Cylinder) xa % 

 falling- perpendicularly on the void fpace in 

 the Cylinder pafl: through it without any Re- 



fraftion s 



