The Optical Magic Lantern Journal (February 1892)

18 The Optical Magio Lantern Journal and Pho‘ographio Enlarger. The Optical Lantern in ClassRoom Work.—1. By Arcupatp Barr, D.Sc., M.Inst.C.E., Regtus Professor of Engineering in the University of Glasgow. THE optical lantern has been extensively adopted for the illustration of popular and public lectures, but it has not yet been very generally applied in ordinary class-room work. This has arisen, no doubt, from a variety of circumstances attending its use, the principal of which are probably these: (1) The necessity for darkening the lecture-room, in order to make the lantern pictures visible; (2) the transient character of the pictures, which renders them unsuitable, in some cases, for class-room use; (3) the necessity which usually exists for employing an assistant to work the lantern, and the consequent difficulty of referring back to an illustration, as it is often desirable to do (in class-teaching; (4) lanterns in every way suitable for class-room use have not been available, at least until lately; (5) supplies of oxygen for the production of the lime-light were not conveniently obtainable until quite recently; and (6) the production of lanternslides in sufficient quantities to illustrate every detail of a subject has been a matter of some difficulty and no little expense. Arrangements made at the Yorkshire College, Leeds, to overcome these difficulties, hive led to a very great extension of lantern illustrations in ordinary class work, and it appeared to the author that a brief description of some of these arrangements might be of interest to those engaged in teaching. The first drawback to the use of the lantern — namely, the necessity for darkening the lectureroom when exhibiting the projected pictures— may very simply be overcome, so much so that during the day-time the room need only be very partially darkened, while in the evening, when the light is less diffused, no darkening whatever is necessary. In the Engineering Lecture Room at Leeds, for example, the projections are shown while sixteen incandescent lamps are lighting up the room quite brightly. The essential condition is simply that the lantern-screen shall be in shadow, when it will be found that an ordinary lime-light lantern will give excellent results, though the room, generally, may in the day-time be moderately lighted, and in the evening may be fully lighted. Briefly, the arrangement is this: The lantern screen, which is placed over the blackboard, is inclined at such an angle, that, while the electric lamps, which are situated near the ceiling, illumi nate the lecture-table, the blackboard, and any diagrams hung onthe walls, no direct light falls upon the screen. By this arrangement lantern pictures, ordinary diagrams, blackboard notes, and models, or experiments can be shown simultaneously. The transient character of the illustrations evidently cannot be overcome, and where it is essential that an illustration should remain continuously before a class for a _ long period, ordinary diagrams must be used. Such cases are, however, very few, because usually the student may have time enough allowed him to sketch such simple illustrations as it is advisable to use in ordinary class work; and if not, he can be referred to some book or journal in which he may find a similar illustration, which he may study again at leisure. The transient character of the illustration is, however, by no means altogether a drawback. Only one illustration being shown at a time, the student’s attention is not distracted by his desire to copy illustrations not at the moment being referred to by the lecturer. The lantern is of little value for ordinary class work, if it is necessary to employ an assistant to work it. By using a short-focus lens, however, the lantern may very conveniently be placed on the lecture-table, and the slides laid out upon a sheet of white paper, so as to be readily seen by the lecturer in the lighted room, and any one may be shown as frequently as may be necessary during a lecture. For ordinary slide work many suitable lanterns are now in the market, and require no description. A suitable lantern for the exhibition of slides and experiments has been a great desideratum, especially for the illustration of lectures on physics, chemistry, physiology, &c., but the want appears to be very fully met by the lantern devised by Professor Stroud and the Rev. Mr. Rendell to meet the requirements of lecturers who desire to ‘exhibit both slides and experiments; but it pos sesses special advantages for the class-room, even when experiments have not to be projected on the screen. As will be seen in the illustration, the lantern is arranged for either horizontal or vertical projection. The sloping mirror, shown in the front part of the lantern, receives the beam from the first lens of the condenser, and reflects it vertically through the second lens, fixed horizontally in the top of the front portion of the lantern, and thence through the objective, which can be moved up and down for focussing by means of a rack and pinion on the pillar. The mirror fixed above the lens projects the beam at any required inclination towards the screen. The lower | mirror is hinged at its upper edge to the front board of the lantern, and, by means ofa stud projecting through the curved slot in the side of the