| The Stereo-motion Orbit of a Machinist's Hand. | Lines of Light Indicating to-and-fro Hand Movements. |
The latest development in Micro-motion Study. A small electric incandescent lamp is attached to the workman's hand, and the lines of light photographed.
There is an increasing tendency to adopt continuous cinematography in preference to the intermittent motion for many other phases of particular study, especially where very fine results are desired. One mechanical engineer has applied the method to the measurement of the deflection of bridges when undergoing tests. Monsieur Deslandres adopted a combination of stylography and chronophotography for recording the vibrations in metal bridges under varying conditions of traffic as far back as 1892, but the direct cinematograph record is to be preferred. Sometimes a camera has been used in combination with the existing processes of observations, so that a photograph of the actual movement and of its extent is obtained simultaneously, while the calibration of the sensitized surface, or the introduction of a clock, like that used by Marey, enables the time intervals to be accurately determined.
Another ingenious form of continuous record, which was made many years ago by Soret and Georges Demeny, the collaborator of Marey, has been revived in an improved form by Mr. Frank B. Gilbreth, the eminent American authority on motion study. In this case a moving film is not absolutely essential, but under certain conditions it is to be preferred. The object of the study is the tracing of motions with a view to their improvement and expedition, or the elimination of unnecessary actions, so that the particular task may be achieved in less time and with reduced exertion.
A stationary plate may be used, and the path of the motion is indicated by a ribbon of light from a small electric incandescent lamp which is attached to the hand or other limb of the subject. In this instance the plate presents an apparent jumble of lines, but by the aid of a magnifying glass the complete cycle of movements can be followed from end to end. When the photographs are taken upon a stationary plate, however, it is necessary that they should be taken stereoscopically, so that relief may be given to the picture to enable the movement to be followed correctly. The hand or limb may not be visible in the photograph, but that is a minor detail, because the path it has described is indicated by the lines of light. When the subject is continually advancing, where it does not double back upon itself, a slowly moving film will supply a complete and perfect graphic record of its progress. But in all such experiments the timing element must be incorporated, or the record will have little practical value, and will provide no conclusive evidence.
A novel application of this method was carried out by Demeny several years ago. The scope of the investigation was the study of the characteristic walks and gaits incidental to certain maladies. Patients suffering from rheumatism and other complaints which interfere with the natural walking motions were taken into a darkened room. Incandescent electric lights were attached to their shoulders, heads, and other parts of their bodies, and these were photographed as the subjects moved about the darkened room. The results upon the sensitized surface were merely the paths described by the moving lights. There have been many applications of the continuous record, especially to the work of testing physical, chemical, or electrical phenomena. The observer is certain to obtain a correct result. This has been shown in certain microscopical observations where the movements are extremely rapid, and where a graphic outline is more important than the photographic detail of the subject.
The great advantage of the continuous cinematographic system is that it records every movement. Even the slightest vibrations will be indicated, and upon a large or small scale according to the rapidity with which the travelling sensitized surface is moved through the camera. Comparative investigation between this and the other systems has proved that the continuous cinematograph shows many motions which the former systems lose. It has introduced the investigator to many curious phenomena of which he was previously ignorant.
One thing must be emphasised. It is the salient difference between the ordinary cinematographic method and the continuously moving film process. The first records upon the film a complete picture of the subject. The second records only the path or trajectory of a single point, or at the utmost of a number of points, of the subject under observation. This remark does not refer to the method of cinematographing with the electric spark, which is a totally different and special application of the art.