3 8 5

Appendix II.

ftess of the Instrument. Also since at thesame Time a very great magnifying Poweris required in the Instrument to which itis applied, it can be applicable for Astro-nomical Purposes in only Reflecting Tele-scopes, because of the Conveniency of adapt-ing it to the Tube of the Telescope, andmoving it at the same time you are observ-ing the Object through it*

V. Concerning the Improvement of RefractingMicroscopes and Telescopes by aCompoftion of Glasses.

I T has been already fhewri in AnnotationCXXVIII. Art. 18. that the Errorsarifng from the different Refrangibility of theRays of Light will be as the Cubes of thelineal Aperture of the Glass direSlly , and theSquares of its Usance inversely ; and conse-quently in Glasses of the same Aperturethe Indistinctness of Vision will be in-versely as their focal Distances ; whereforeif, while the magnifying Power and Aper-ture of the Lens continue the same, wecan increase this Ratio of the focal Dis-tance to the said lineal Aperture, we shallin that Proportion increase the Distinctnessof Vision, which is the very Point in which,the Perfection of Microscopes and Telescopesconsist.

Vol. III. C c

To