The Optical Magic Lantern Journal (October 1889)

36 The Optical Magio Lantern Journal and Photographic Enlarger. with O2, and Hz with Hz. The other outlets are each connected to the jets thus—Hr and O1 to top jet ; H3 and 03 to middle jet ; and H4 and O4 to the bottom jet. The straight handle is termed the coupler, and the bent one the dissolver. According to the angle at which the coupler is set, so are the different lanterns coupled. With this dissolver the bye-passes are each arranged from asmall cock in the service pipe and are not included in the tap itself. Another style of tap which needs little explanation is one which is furnished with a dial. In this case the index handle is merely moved to the required combination. Complex as many of these dissolvers may appear, yet when fitted on the lantern it is surprising with what ease and certainty they are manipulated. Of course the dissolving tap may be placed almost anywhere, but for convenience sake it is generally attached tothe lantern itself at the back, that position being found to be preferable, as it is out of the way of being accidentally moved. When one’s resources will not permit of twoor more lanterns, and it is not the intention to exhibit effects, it is quite possible to produce upon the screen a dissolving picture. This may be done by procuring a strip of clear glass, and converting a portion of it into ground ‘glass. The Junction between the clear portion and the ground part must be gradually vignetted; the ground glass portion may then be stained with transparent oil colour of a neutral tint, the colours being laid on thinner and thinner as they approach the clear end of the glass. This strip is made to slide in suitable grooves close in front of the objcctive. When in use the rays of light pass through the clear portion, and by slowly sliding the glass upwards a mistiness will appear in the foreground of the picture, which will by degrees envelope the whole view ; when the slide can be changed, and gradually brought to view by reversing the action. The neutral tint is of assistance in relieving the eyes ; by having a clear portion of glass in front of the objective there will be no sharp line of demarkation ; and by commencing the dissolving at the bottom of the picture a better effect will be produced than if it were begun at the top, for in the majority of cascs the foreground is the darkest part of the picture. To Ascertain the Candle Power of a Light, MANY are in the habit of speaking of a light as being of so many candle-power, but if asked how they ascertained it, the answer will in many cases be that it was a rough guess. If an object intervene between a white screen and a light, no matter of what intensity, a shadow must necessarily be cast upon the screen. If the light employed be a strong one, the shadow cast will be more intense than if the light had been of léss brilliancy. As light loses a great deal of its force the farther it is removed, so the farther the light is from the object spoken of, the less dense will be the shadow cast, such loss being the square of the distance. As is well known, a candle-power is the light ofa standard candle, Let it now be supposed that it is desirous that the candle-power of a given light is to be ascertained. How must we proceed? In order todo soit is requisite that we ascertain the strength of one candle-power, which is best done by placing a rod a little distance in front of a white screen and allowing the shadow of the rod from a candle placed a little distance beyond to fall upon the screen; this shadow then represents one candle-power. We next consider the light to be tested. If it be more intense than the one candle, the shadow of the rod cast from it will be deeper, both lights being placed at thesame distance. Now move the light that is being tested to such a distance from the rod that the shadows assimilate in density, which may easily be ascertained when they are side by side. The intensity of the two lights will be directly proportional to the squares of the distance from the shadows. Thus, suppose the light to be tested is three times the distance of the candle, its illuminating power will be nine times as great, and the ascertained candle-power of the light will be nine candle-power. It is impossible to give with definite accuracy the relative intensities of the various lights employed in the lantern, as so much depends on the forms of burner and the skill of the operator. For example, the Solar Fountain Lamp equals from twenty to forty candles, according to the diameter of the flame, the Purity of the oil, and the perfect adaptation of the chimney. Lamps of the Sciopticon class, according to the number and length of their wicks, will vary from thirty to seventy-five candle-power, sometimes even exceeding this figure. The Spirit or Oxy-calcium light is by very many considered to range much lower than the ordinary blow-through or safety gas Jet, but this arises from the iinperfection of construction and not from any inherent defect in the principle of illumination, not but that it is slightly inferior to the other, but not to the extent