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presence of a small amount of A16-20-acetoxy material may account, in part, for the difficulties encountered in obtaining crystalline material.A similar acid-catalyzed equilibration of a 1:1 mixture of 20aand 2O0-hydroxy-A16-3-ethylene ketals10 afforded the same mixture of products in the same relative proportions as were isolated from the pure 20a isomer.16a-Hydroxy-4,l7( 20)-(irans)-pregnadien-3-one (13a).-The treatment of the 16<*-acetate 13b with alcoholic potassium hydroxide at reflux for 1.5 hr. afforded the 16a-hydroxy compound (13a). The product crystallized as needles from ether-petroleum ether (b.p. 66-69°) and had m.p. 173.5-175.5°; [a]n +104.5°;

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The attainment of an effective level of laboratory instruction poses many interesting challenges to the instructor in that students need to be evaluated on such diverse matters as manipulative skills, data aquisition, and interpretive abilities. The most important characteristics of any laboratory exercise lie in the student's ability to successfully carry out the procedure while gaining a clear understanding of the theory on which the experimental procedure is based. To simplify the transition that the chemistry student must make-in putting classroom theory into laboratory practice-pre-laboratory instruction has become an important element of undergraduate chemistry.Pre-laboratory instructional programs tend to vary in their scheduling formats and content emphasis. The direct approach is the traditional short lecture presented a t the start of the laboratory period. Here, theory relevant to the experiment is reviewed-time nermittine. But eenerallv. the em--phaiis is placed on disa~ssim of laboratory proredures directly related to the en~eriment. Both time limitations and class size often preclude effective pre-lab demonstrations of the exneriment. A serious disadvantaee of this instructional format is its infringement on scheduled laboratory time. As an alternative, actual course lecture time can be set aside each week for laboratory orientation. However, if discussion of the laboratory exercise coincides with the first class meeting of the week, two or three days may elapse before students carry out the exoeriment in the laboratorv-a considerable disadvantage frim an instructional point of view.While all of these formats are nominally effective, they uniformly suffer from an old idea: in order for a laboratory exercise to become a meaningful educational experience, students must' be made to grope and stumble to a proper conclusion on their own. While there are important lessons to be learned in unaided discoverv. it is doubtful that facultv members can continue to indulge cn'these same tactics because of increasing demands for classroom efficiency and instructional accountability. We believe that the use of audiovisual technoloev nrovides an excellent means of workine toward -" .those goals in that a maximum of information can be made available to the student in a minimum of time.The pre-laboratory programs that we have developed during the past six years provide teacher-produced instructional materials for use by students and faculty in coniunction with a variety of media delivery systems. The focus bf the media component for these pre-laboratory programs are sets of highly registered 35mm slides for use with twin slide projectors, alternately activated by a dissolve control unit. This delivery technique, known asLap-Dissolve Projection, has nroven to be a remarkablv effective means of demonstrating chemical concepts through controlled motion for a erowing audience of chemistrv instructors (1-5).-We wish to describe the use of ~a~-~i s s o l v e materials to prepare self-pacing instructional programs that stud...