As is done in our Author's Lect. Optic. Part I. Sect. III. and IV. and Part II. Sect. II.

See our Author's Lect. Optic. Part II. Sect. II. pag. 269, &c.

This is demonstrated in our Author's Lect. Optic. Part I. Sect. IV. Prop. 35 and 36.