Viewing page 235 of 504

This transcription has been completed. Contact us with corrections.

[[right justified]]44)[[right justified]]
But If the Quadrant should be already graduated the point O, or 45 degrees, must be found as above and the distance each falls from those put upon the Quadrant must be allowed for in every observation. [[underline]] Bion [[/underline]]
   "[[underline]] Honoratus Fabri [[/underline]] in his [[underline]] Synopsis Optica [[/underline]], says, That [[underline]] Eustachius Divini [[/underline]], a famous [[underline]] Roman [[/underline]] Optic-glass maker, made the Eye Lens of his Telescope to consist of two equal, narrow plano-convex-lenses, touching one anothers convexities in the axis, and so placed, that the center of the plano-convex-lens next to the object lens, was in the Focus of the object lens; by which means the Rays that came parallel from the object, would fall parallel upon the Eye: and says [[underline]] Fabri [[/underline]], some of the advantages of this Telescope are, that the colours of the rain-bow are excluded from it. The Angle of the Sight augmented. A greater field is taken in at one view. The Object appears more lively and bright. Lastly he would have water included in the vacuity between the concavities of the two touching plano-convex-eye-lenses. See much more of this in 46 Trops. of [[underline]] Fabri [[/underline]]'s Optics." Stone's Transl. of Bion. ^[[insertion]] Append. [[/insertion]] p. 280.
   "if two equal lenses be joined together so as to touch, the Focus will be removed to half the Distance of one of them"; these with a proper charge do well for an Eye-glass. Stone's tr. of Bion. ^[[insertion]] Appendix [[/insertion]] p.280.
   "Multiply the Number of Feet in the Focal distance of the object lens by,3, and the Square Root of the product will give the Diameter of the Aperture (for the object Glass) in Inches and Decimal parts: and the same augmented by a tenth Part of itself, will be the focal distance of the Eye-lens; and the apparent Breadths of the objects are as the Diameter of the apertures. Ex. gr. If the focal Distance of the object lens be 30 Feet it will thence be found that the Diameter of the Aperture of the object lens will be 3 inches. The focal Distance of the ocular lens 3,3 and the Proportion of magnifying considered as to the Diameter in this Telescope to One of an Object lens of one foot - focus is as 109 to 20." Bion's Appendix p. 281. [[insertion, bracketed]] From the given Magnifying power of 109 to 20 I find that the aperture of the Eye glass = ,55 of an Inch. [[/insertion]] [[/bracket]]
[[insertion under the line]] Also Smith's optics Vol.I. p.149. Art.355. [[end insertion below line]]
   [[underline]] Mr. Hu ^[[insertion]]y[[/insertion]] gens [[/underline]] says the focal Distance of [[strikethrough]] a [[/strikethrough]] ^[[insertion]] all [[/insertion]] double convex lenses are in proportion to the Radius of the Spherical Surface as 11 to 12. Bion. Append. p. 282. V. Simpson's Algebra 2.[[superscript]]d[[superscript]] Ed. Cor. 2. p. 314. [[insertion]] Also. N.[[superscript]]o[[superscript]] 205 of Philos. Trans. Or Vol.I. p.183 of Lowthorp's abridgment. [[/insertion]]
   "That Light descreases as the Square of the Distance, I am doubtful of, and have been so many years; there is no proof of it by acutual Experiment as I know: indeed it has been long made out by Theory to be so; but the practical Proof of these things is best, and most to be relied upon; and I have often thought, that Light in some Cases, as well as Heat, may decrease, rather as the Cubes of the Distances, than as the Squares." Ed. Stone, in Bion, App. p.288.

Transcription Notes:
Several instances of the old-style long S - I don't know how to indicate this. See especially the word "glass." -I believe the convention we are following is to transcribe them both as modern "s" to enable searching in the page Also the "," before a number indicates a decimal and should be immediately before the number -megshu