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The Optical Magic Lantern Journal and Photographic Enlarger. 83
Some Improvements in Optical Projection.
TNO) URING the past few years, Mr.
‘j J. H. Edgar Baugh has made numerous experiments in order to improve the optical lantern, and upon this subject some time ago gave a lecture, and also exhibited his new form of lantern before the Royal Photographic Society. Mr. Baugh’s aim had been not so much to obtain the maximum’ portability, but rather the greatest efficiency.
The lantern which he exhibited consisted of a baseboard, to which was secured the body proper; whilst on suitable runners the two uprights for holding the slide and objective could be adjusted at any distance from the body, and readily clamped in the desired position.
In every optical instrument (Mr. Baugh said) rigidity is most essential. ‘bere must be no topheaviness or strain anywhere. I strongly fecl that the average commercial lantern with the brass tubes holding the projection lens supported from the body of the lens is very much wanting in rigidity. In the lantern I have brought to show you the baseboard is 3 feet Jong, which length is sufficient to hold the longest focus projection lens which I have ever wanted to use. This baseboard is made of a piece of mahogany strengthened at each end, and is very rigid.
The body of the lantern must be rnade of metal, because it has to support a considerable amount of heat, especially when the
arc light is used.
The mahogany external body seems to me to be of very little use. It was said to prevent one’s fingers from getting burnt, but I have not yet experienced an accident of this sort. A thin Russian iron body cools after use fairly quickly. A large door on either side enables one to get at any part of the jet or lamp. When examining the jet in a dark room it is very couvenient to have two doors, as then more light is let in and the lantern is more easy to work from either side.
The usual method of mounting the condenser is to place it entirely inside the lantern, so that it can only be removed for cleaning by removing the jet, etc. It seems to me very much better to mount it half inside and half outside the lantern, so that it can be removed from the
front of the lantern instead of from the inside.
For ventilation and for cooling the glasses a sufficient number of holes must be bored through the mount of the condenser and through the tube which supports the same.
I will next draw attention to the carrier. It is generally admitted, I think, that it is tiring to the eyes if between one slide being shown and the next there is a dark interval, or if the slide during the process of changing goes out of focus. For ease in working, simplicity and pleasantness of effect, the ordinary push-through carrier is most efficient. It has, however, one great drawback, and that is, tle slide is usually liable to wobble. Several years ago I tried to get springs fitted to obviate this difficulty, but never was able to get them to work. I have to thank Mr. EH. T. Martin, of the Northampton Institute, for designing the form of spring fitted to the instrument. The slide is firmly held while in front of the condenser, and when pushed on one side for removal is loosed and raised so that it can be easily caught hold of.
The usual method of holding the carrier is to box it in between the condenser and the tube holding the projection lens. It is then held with springs and has to be readjusted as to centering, etc., every time the lantern is used. Owing to the boxing in, the slide
gets very hot,
which is not at all desirable. The form I tave to show you is entirely different. The carrier rotates and is centered once for all, and can never get out of centre. The picture can be levelled in a moment by simply turning a screw. The slide does not get unduly heated, because there is an ample current of air both sides. The carrier support is fitted with rack and pinion. This serves to place the slide at exactly the right distance from the condenser. It is, moreover, very useful when using long focus projection lenses, as it is much nearer the operator and can be used for focusing to the extent of the difference between the different thicknesses of slides used. The lens support slides along the baseboard and can be clamped tightly. Itis fitted with lens boards, so that different lenses can be used.
'I will now draw attention to the optical portion. I do not think we have reached the limit of perfection in condensers yet. I use a Voigtlinder one which has been corrected to give a remarkably perfect parallel beam. It is made of specially annealed glass and improves the definition on the screen. Regarding projection lenses, I have made a number of experiments with lenses of nearly every type. I find that when considering projection lenses of