SOME WATSON APOCHROMATIC MICROSCOPE OBJECTIVES:

c. 1910

Maker: W. WATSON & SONS LTD.

Four classes of objectives were available at, or near the time of the manufacture of the Van Heurck microscope about 1910. These included ordinary Achromatic, Parachromatic, Holoscopic, and, Apochromatic types, in increasing order of resolution and cost. The Apochromatics were the most highly corrected objectives corrected not only for spherical aberration but were also highly color corrected. At the time Watson started to make Apochromatic objectives, flat field (plan) Apochromatic objectives could not be produced. Watson initially supplied Apos by other makers but by the latter part of the second decade of the 20th century, started to supply their own Apos.

The Apochromatic objectives with my Van Heruck microscope at this time include the 4 mm (¹/₆ inch), and the oil immersion 2 mm (¹/₁₂ inch). Note that the 4 mm is earlier, and was configured for the then Watson Standard tube length of 200 mm. The later 2 mm oil Apo was configured for the later RMS and European standard tube length of 160 mm.