PREFACE.

concise or natural as was necessary, I haveproposed it in my own way, to answer thisimportant End : For the greatest Excellencyof a Demonstration is Conciseness and Perspi-cuity. The Want of this is a great Discou-ragement to Physical Enquiries, as I am buttoo well convinced of by my own Experience,and that of many others.

Fifthly, According to the differentNature of the Subject, a various Process hasbeen made use of to explain or illustrate thesame. Thus some Things require a Phyfcalyothers a Mechanical , some a Geometrical , andsome an Experimental Proofs which havebeen accordingly applied, sometimes singly,sometimes two or more, and sometimes alto-gether. Thus from Phœnomena we institutea Ratiocination concerning the Materiality,the Velocity, different Magnitudes, Colours,and other Affections of the Particles of Light.Then the Laws of Reflection and Refractionare explained by Mechanical Principles relat-ing to the Motion and Action of Bodies oneupon another. Again, The Principles ofGeometry are call’d in to explain and accountfor the Effects of Mirrours, Lenses, OpticInstruments, the Rainbow, &c. And, Lastly*those various Phænomena are represented,and the Manner how they happen explainedby Experiments.

A 4

Sixthly,