The Optical Magic Lantern Journal (February 1897)

30 The Optical Magic Lantern Journal and Photographic Enlarger. attention or hurried manipulation of these parts. Ignorant Operators sometimes set the jet hissing loudly in the vain attempt to produce a brighter disc, which, besides attracting the attention of the audience, annoys the speaker considerably, whose remarks are often drowned by the piercing sound. The lanterns suitable for public use are now so perfect that, if properly managed, their faults are imperceptible. Although on many occasions double and triple lanterns are employed, much good work can be done by a single lantern, but I would advise those who use this form not to attempt or expect too much from their instruments. As far as showing efficiency goes, the single lantern is as useful as the bi-unial or triple, but if dissolving or effects are to be produced, it is much wiser, and in many cases _absolutely necessary, to employ more than one lantern. Now a word regarding slides suitable for exhibitions of this class. They should, of course, be as technically perfect as possible, and if pictorially so, all the better. In cases where possible, this latter quality should always be present. Even the eye not trained to know a good picture can tell when the view is pleasing or not, on account of its arrangement or pictorial composition. The slides should be free from complication, and should be of a nature to be readily understood by the spectators. It should not be necessary for the lecturer to stop his discourse to point out something of interest in the slides, as there should only be one principal object in the view, and that should not require explanation. If any subordinate part of the picture deserves special attention, it is generally as well, if possible, to have a slide of that part alone, which can be shown after the principal view. Instead of having to explain the slides, the lecturer should have the slides explaining him. That really is the main object of having the lantern at lectures at all, namely, to let the audience have a better idea of the subject or place spoken of, by showing them pictures of the same. It too often seems that the slides are the principal attraction, and the lecturer a mere accessory to the entertainment, or, as it then becomes, show of slides. Theoretically, at a lecture pure and simple, no rotice should be taken of the screen by the lecturer, which should illustrate, as the pictures of a book do the matter accompanying them, the lecturer’s ideas and expressions. In conclusion, let me suggest the following rules for the successful management of highclass lectures. 1. Be sure the lantern is firmly placed so as to avoid a shaky picture on the screen. 2. Be sure the disc will not be encroached upon by the heads of the audience. 3. Do not ask to be “favoured with darkness,” but have the hall-keeper ready to turn the lights out at a secret signal from yourself. 4. Do not show any slide which requires an apology on account of bad photography, etc. 5. Although a few effects are good, too many should be avoided. 6. Do not attract attention by remarks to the operator during the lecture. 7. Never point yourself out in a picture. 8. Do not make too many jokes. 9. Do not show too many slides. (See Maaic Lantern Journat for Jan., 1894, p. 15.) ay as Is Acetylene Poisonous? Precautions to be taken with this Gas. OW that acetylene is becoming more common as an illuminant, a French scientist, Dr. N. Gréhant, considers it a work of utility to acquaint engineers and manufacturers with a number of experiments he made during last year. These re searches are divided as follows :— I. Physiological researches; poisonousness of this gas. II. Complete combustion of analysis of products of combustion. III. Incomplete combustion of acetylene ; production of oxide of carbon. Comparison with the incomplete products of combustion of lighting gas. IV. Explosive mixtures of air and acetylene ; explosive mixtures of air and lighting gas. V. Brilliancy of acetylene flame ; hygiene of the eye. I. Puysrotogican ResrarcuEes. — Thirty years ago, the eminent chemist, Mr. Berthelot and Claude Bernard found that sparrows kept in an atmosphere mixed with some ;3,_ of pure acetylene did not seem to be affected in any marked way. In 1887, a Mr. Brociner, of Bucharest, maintained a thesis, at the Parisian School of Pharmacy, about the poisonousness of acetylene. Mr. Brociner studied the effects on a guinea-pig of 99 volumes of air and 1 of acetylene; 95 of air and 5 of acetylene; 90 of air and 10 of acetylene; and 50 of oxygen and acetylene ;