Appendix II.

Lenses, they will likewise be in Contactin the shortened compound Focus. And asthe Centres N and O of the two Sett 1 ' 1 "Lenses G E and I H are separated farth erfrom, or brought nearer to each other,Images in either Focus will be moved ^similar Manner ; and when the Centres *and O coincide, the Images in each Fo cllSrespectively will also coincide, or b tco^one entire Image, the Difference in eveiTCase being only as to large and small, grea teror lesser Distance. Consequently inMicrometer by which those two Sett 11Lenses are moved by each other, the s arsl<!Turns of the Screw which measuresAngle O P N, and which brings the Im a &into an exact Contact in the single Fo° llSat will be necessary for the fame P uspose in the compound Focus also; so t , 'by this Means we have an Opportun 1 Jof measuring the said Angle O P Q^. ^

out being obliged to have so greatso unmanageable a, Length of the * escope.

However, the larger the focal Did arl f^pf the Lens A B is, the more distinct ^Contact of the Images will appear;because this is the Point on whichwhole Perfection of this Micrometer

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