exhibited by thin plates. 18 1 
principles of physical optics, unless perhaps the newly dis- 
covered principle, which „has been denominated the polarity 
of the rays of light, may serve to explain their causes. 
However, “ as every desideratum may be considered as an 
imperfect discovery/’ which when completed may lead to 
others of still greater importance ; and as the subject is so 
far from being exhausted, that perhaps we are only entering 
on a new career of discoveries, not only in optics, but in 
every other branch of natural knowledge ; in this point of 
view, the prospect before us is rendered as extensive as it is 
animating and delightful. 
JOHN KNOX. 
Belfast , Oct. 6 , 1812 . 
