C hap. 4.3 ef the Rays of Light. 2 1 $ 



have been juft introduced to the notice of the reader 

 will be further explained and elucidated -, but as the 

 application of the general doctrines of reflection and 

 refraction to optical fcience can only be underftood 

 upon mathematical principles, I have thought it pro- 

 per to diftinguifh thofe chapters by the fcientific and 

 technical words, catoptrics and dioptrics. In the mean 

 time, the majority of readers will find fufficient to la- 

 tisfy their curiofity, and to afford them a general view 

 of the nature and effects of this wonderful fluid in the 

 preceding obfervations, and they may therefore pro- 

 ceed immediately to the feventh chapter, which treats 

 of vifion and optical glares. 



