Abstract: In an optical pickup apparatus which can be commonly used for a plurality of optical disks having different thickness values, thickness t of a optical transparent plane plate for thickness compensation is set by means of skew ray formula so that the amount of spherical aberration caused by the optical system is lowered, thus allowing accurate focusing for each of a plurality of optical disks having different thickness values. Spherical aberration S.A. is given by expression of S.A..ident.S'.sub.k -S'.sub.k0, wherein S'.sub.k is computed by expression of S'.sub.k =Ax.sub.k -(X'.sub.k /Y'.sub.k)Ay.sub.k. Here, by setting S'.sub.k -S'.sub.k0 =minimal value, the geometric thickness of the plane plate 8 can be determined.

Abstract: An overhead projector for projecting an image of a sheet material placed on a flat stage onto a remote screen standing almost vertically has a projection optical system with its optical axis aligned with a normal line of the stage. The projection optical system is shifted keeping the optical axis in parallel with the normal line so as to project an image at an angle of elevation onto the screen.

Abstract: A gas purification system has: a first treatment apparatus, which purifies an untreated feed gas, containing toxic ingredients such as HCN gas, H.sub.2 S gas and NH.sub.3 gas, into a primary treated gas by causing the feed gas to contact with an alkaline absorbent liquid; an absorbent-liquid-resuscitation apparatus, which refreshes the absorbent liquid, after absorbing the toxic ingredients, by bubblingly introducing a resuscitation gas; and a second treatment apparatus, which further purifies the primary treated gas into a secondary treated gas by strippingly capturing any possible remnant NH.sub.3 gas still present in the former. The resuscitation gas and the toxic-ingredient-absorbed absorbent liquid are forcibly converged by premix nozzles into combined gas/liquid jets as are injected into the absorbent-liquid-resuscitation apparatus.

Abstract: A gas purification system has: a first treatment apparatus, which purifies an untreated feed gas, containing toxic ingredients such as HCN gas, H.sub.2 S gas and NH.sub.3 gas, into a primary treated gas by causing the feed gas to contact with an alkaline absorbent liquid; an absorbent-liquid-resuscitation apparatus, which refreshes the absorbent liquid, after absorbing the toxic ingredients, by bubblingly introducing a resuscitation gas; and a second treatment apparatus, which further purifies the primary treated gas into a secondary treated gas by strippingly capturing any possible remnant NH.sub.3 gas still present in the former. The resuscitation gas and the toxic-ingredient-absorbed absorbent liquid are forcibly converged by premix nozzles into combined gas/liquid jets as are injected into the absorbent-liquid-resuscitation apparatus.

Abstract: An optical system for a color television camera includes an interchangeable taking lens, a color separating prism block located behind the taking lens and three image pick-up tubes for different color images located behind the prism block. Between the prism block and at least one of the pick-up tubes an optical element which is capable of varying the optical path length is inserted. The optical path length varying element may be a plane parallel plate or a combination of two optical wedges having parallel opposite end faces. The plane parallel plate changes the optical path length by its thickness. The combination of the two optical wedges varies the optical path length by sliding the wedges with respect to each other to vary the distance between the opposite end faces.