M. T. D. 'rissandier has devised a simple apparatus which is still more effective. This apparatus consists of a rotating disc of some white substance having its face smoked to blacken it, and a spring attached to the disc, the end of which is a fine needle having a slight pressure upon the disc sufficient to remove the black coating The spring is put in motion by the leves at the bottom shown in Fig. 395. To measure a shutter the instrument is set up facing the camera in a strong light—sunlight if possible— the lever is worked and the spring commences to vibrate. The disc is then set in motion and the result of a short exposure is shown in Fig. 396, the needle attached to the spring making the curves as shown. If the speed of vibration of the spring and the rotation the disc be known it is easy to calculate the speed of the shutter by the length of the sinuous lines which were photographed on the plate. If, however, the time of exposure be greater than that required for the disc to make one revolution, it is not a difficult matter to so modify the instrument that it will record the exposure in the form of a spiral as shown in Fig. 397.
Another apparatus has been devised for this purpose by Addenbrooke. A sensitive plate is caused to revolve at a given speed behind a small opening made in an opaque screen, in front of which magnesium ribbon is burnt. The shutter is fixed between the light and the aperture of the screen. The proportion that the curved line of the developed image bears to the whole circle is the proportion that exists between the time of one revolution of the plate and the expo sure given by the shutter. For example, if the plate makes one revolution per second, and a tenth part of a circle is produced on development, it is apparent that the duration of time the light was admitted was the one-tenth of a sec on d.
In photographing moving objects it is use ful to know the maximum amount of exposure that can be given at different distances. The following calculations of W. Groves* will enable one to do this satisfactorily : Let x = distance of image from lens (i.e , focal length) in feet ; y itobject " in feet ; d— amount of movement of object permissible in feet ; v — velocity of object transversely to axis of lens in feet.
Assuming that the amount of movement of the image on the plate permissible is inch, that is, foot, we have d — y. YPM - • , 1200 XNow, -(7 7' - the number of times the object traverses the permissible distance in each second; .•. Time lens may be uncovered (speed of shutter) — d y 7, sec. = — sec. — secsIf 120027/ Examples •— If .r — 8' (i foot), and y — io feet, and v feet per sec., then 10 Speed of shutter — noo + I + 6—o3 secs.
If y — 20 feet speed — = sec.
If y — 3o feet " = —- sec.
suo x20 With many shutters very little effect is produced on the plate during the periods near opening and closing, and it is probable that the minimum time of exposure in such cases might be one-fourth or one-third more than the above without visible blurring.
Captain W. de W. Abney explained before the Camera Club, of Loudon, a short time ago, his method of measuring the speed of photographic camera shutters, which has special advantages as regards accuracy and facility of record, brought about in a somewhat novel manner. In a report of his lecture, which we extract from the London Amateur Photographer,f are several in teresting facts. The lecturer pointed out that it was quite as important to know whether we
were giving an exposure of. say, or of a second as one of 5 or 15 seconds. The apparatus enables us not only to measure the time of exposure, but also causes any kind of shutter to draw its own diagram, and from this diagram several things are made known. c. g., how long it took to open, and to close, and how long the working aperture of the lens was fully open, etc.—three points of very great practical importance. The apparatus employed is somewhat as follows: A source of light; in this case the electric arc, but magnesium can be used; the essentials being a steady and strong actinic light.
A supplementary positive lens. This is so placed that it throws an image of the carbon points upon the lens in the front of the camera.
The electric arc lamp will be noticed at the right hand of the end of the engraving, which projects a beam of light upon the condensing lens supported on a stand, and this in turn concen trates the beam upon the shutter to be tested, which is held in an upright stand next to its left. The actuating bulb of the shutter will be seen upon the table. Different makes of shutters can be held by this stand. The stand next on the left supports a spectroscope tube without any lens, having the slit two inches long by of an inch wide, in a horizontal position. A cardboard with a slit cut in it, inserted in the as well as the'regular slit. The coadensing:lens is adjusted with reference to the light so as to fully cover the whole of the horizontal slit. The motion of the shutter is in the direction of the length of the slit in the card.
At the left of the spectroscope stand is a rotating circular cardboard disk divided into six sector openings divided only by a narrow radial bar. The apparatus reminds one of a wheel with only six spokes. Along the rim are punched out a series of small holes, equidistant. Six of these holes correspond to each sector opening, so that there are thirty six holes in all. The ap paratus is made to revolve (about its center) in a vertical plane just in front of the lens of the camera, and as each spoke of the wheel passes in front of the lens, and is parallel to the slit in the tube, it intercepts the light. The wheel is made to revolve at a uniform speed by a small electro-motor which will be seen to the left of it. It is important to know the wheel's rate of revolution. This may be done in two ways. First, by blowing air through a small tube perpen dicular to the plane of the sector and just opposite the row of the thirty-six holes, it becomes (effectively) a siren. The pitch of the note gives the number of air puffs passing through the holes, and so the rate of revolution is known. For example, suppose the air puffs gave a note agreeing with a tuning-fork which was known to vibrate 720 per second, we should know that 720 air puffs had passed through the tube and holes opposite in a second. Dividing this number by the number of holes in the rim, viz., thirty-six, we get twenty complete revolutions per second, and since there are six spokes in the wheel, one spoke would follow its neighbor in front of the lens in of a second.