286

Of Light and CoLoukg.

reflected to the Eye, which render it visible?the other Part is transmitted, and variously

reflected

Area of the solas Spot^ or the focal Distancej that i s >

P will be as , 4 -; orP : 6 :: =L • —- f 2 : F 2 .t r b 2 ' f 2

5. Consequently when neither the Area ds the Glastnor focal Distance is given* we have the Power otBurning compounded of the direct Ratio of the A^and inverse Ratio of the Square of the focal Distance 0the Glass; or we have P : p :: A f 2 : a F 2 ; which &the Rule above laid down.

6. It has been shewn ( Annot . XCIII.) thai the H e3 *jof Wood Fire is about 35 times greater than that 0*the Summer-Sun (because it raises the Fluid in th 6Thermometer 35 times higher nearly); therefore th^a Glass may be able to condense the Rays sufticientyto burn, or to have the Heat of common Fire, the SunImage, or solar Spot in the Focus, ought to be at &°,but x. Part of the Area of the Glass j and as rhuch as 1is less than a -jV-Part of the Glass, so much the strongs

will it burn. In this Case, if it be desired to know

in

what Part of the Pencil of Rays the Density is 35 t>n^greater than the common Density, and where the P . fof Burning is equal to that of common Fire, st ,found as in the following Example. Admit a (d® ’be 9 Inches in Diameter, and let the Diameter vs £ ^required Circle be (a) ; then since circular Areas are ^the Squares of their Diameters, Wd have 35 : 1 :: 9 1 : a ’

Consequently —- a- a 1 , and so a zu — 1,5 ncL-^D

whence that Part of the Cone or Pencil of Rays, vv'h 0 ^-Diameter is I 1 -Inches, has the Density and PowerBurning required ; and that this is Fact, and that ^Density Of the Rays but a little less than that wist st.,burn, I know from repeated Trials with, such a (d 3 ’or concave Mirrour. _

7. Of Burning-Glasses we have some extraordn) KInstances and surprizing Accounts of their prodigy* EstcSs'