Tube Length Cover Class

Tube Length Cover Class.

TUBE LENGTH AND COVER CLASS

All objectives are corrected to a certain tube length (i6omm by most makers Leitz i/omm) and all objectives in fixed mounts of over .70 N. A. are corrected to a definite thickness of cover glass as well. (Zeiss .I5mm .2Omm; Leitz, .I7mm; Bausch & Lomb and Spencer, .i8mm). These objectives give their best results only when used with the cover glass and tube length for which they are corrected. As indicated in the frontis piece, the tube length extends from the eye lens of the eyepiece to the end of the tube into which the objective or nose piece is screwed. If a nosepiece is used the draw tube must be correspondingly shortened. If the cover glass is thinner than that for which the objective is corrected the tube must be lengthened to obtain best results; thicker, shortened. The more expensive objectives are provided with adjustable mounts by which the distances between the lens systems may be changed to compensate for difference of thickness of cover. They are successfully used only in the hands of an expert. One of them out of adjustment is worse than an ordinary objective.

---