ht line, but the tropiques andother circles of declination wilbe fe&ion ^hyperbolical!, and]>e thus defcribed. . Confider the length of the ftyle, the declination of theparallell, and th* angle at th* pole bctweene the fubftylarand the houre-line, whereon you meane to defcribe thepa- If you… … As in the example of the polar plane, where the lengthof the ftyle B C was found to be 1 inch 61 cent, if you de-fire to know the diftance betweene the equinot~tiall and thetropique vpon the fubftylar line:extend the compares fromthe tangent of q$gr. vnto the tangent of 23 gr. 30 m. thefame extent…
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![An illustration from L'optique divisée en trois livres : ou, l'On démontre d'une maniere aisée tout ce qui regarde 1. la propagation & les proprietez de la lumiere. 2. La vision. 3. La figure & la disposition des verres qui servent à la perfefectionner [sic] (1682)](https://img.insideoutlibrary.com/img/200/ia-loptiquedivise00ango-77-8.webp)
