From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Article on the principles of streak photography, alongside advertisements for Super Sport Dolly and Regular S.S. Dolly cameras.
| Identifier | ExFiles\Box 143\3\ scan0193 | |
| Date | 1st April 1940 | |
| 74 POPULAR PHOTOGRAPHY April, 1940 SUPER SPORT DOLLY With coupled range-finder plus built-in exposure meter... All the features usually associated with more expensive cameras, plus a choice of two practical, economical picture sizes. Takes either 16 pictures 1 5/8x2 1/4 inches, or 12 pictures 2 1/4x2 1/4 inches, on standard 120 film. Has visual type exposure meter and split image synchronized range finder. Leather bellows, self-erecting front, optical eye level view finder, leveling piece and hyperfocal distance table etched on back, are among its many advanced features. With Schneider Xenar f2.8 in Regular Compur delayed-action shutter, speeds up to 1/250, only..................$60.00 With Zeiss Tessar f2.8 in Regular Compur delayed-action shutter, speeds up to 1/250, only........................$70.00 Above models with Compur Rapid Shutter, speeds up to 1/400....$5.00 additional REGULAR S. S. DOLLY Has most of the features of above model except range finder and exposure meter. Focus obtained by front-lens rotation. Fast lens equipment set in delayed-action Compur shutter with speeds up to 1/250. With Meyer Trioplan f2.9 $40.00 With Schneider Xenar f2.8 $47.50 With Zeiss Tessar f2.8..$55.00 At leading dealers everywhere BURLEIGH BROOKS INC. PHOTO GOODS 126 W. 42nd. STREET NEW YORK CITY Photographing the Invisible (Continued from page 16) strated when a stick is partly immersed in water. It then appears to be bent or broken. Now, in our first example mentioned above, warm air forms eddies above the hot rails as it rises, because hot air is less dense than cold air. And because the density of the rising warm air is constantly being changed by cool air currents pressing from the sides, a beam of light, rising from any point on the rail, has to pass alternately through layers of warm and cold air, which means that it is continually more or less refracted. Fig. 1. Diagram showing principle of streak photography, as explained fully in text. This has the effect of making a section of rail appear to be bent either up or down, or to the right or left, depending upon the relative densities at the different points in our field of view. The apparent motion of the distorted image of the rail is due to the motion of the air layers as they ascend or are blown aside by other air currents. The higher the temperature, the greater the velocity of the heated air currents, and the more rapid will be the density changes and the rapidity of the flicker image motion. It is important to note that this phenomenon is apparent only in direct sunlight or in very strong light, even though the temperature differences causing the air currents are just as great with diffused light. The light beam must consist of concentrated rays either radiating or converging from a definite common point, as will be explained later. The perception of the distortion or flickering by the eye of the observer bears a direct relation to the size of the opening of the iris in the eye. In very strong sunlight, the iris of the eye is closed to pinpoint dimensions and it is then that the effect is most pronounced. In dull, diffused light the iris of the eye is larger, and the effect is almost if not entirely lost. The optimum distortion exists only when the iris is very slightly open so that only the refracted rays reach the retina, the rays from surrounding non-refracting areas being filtered out by the iris. The second example of differential refraction is the distortion of a distant chimney as seen through a window pane of unequal thickness. Cheap window glass may vary in thickness from point to point over its surface, or the chemical composition of the glass may not be uniform throughout so that the refractive index, or ability to deflect the light rays by refraction, may vary. These differences in refractive ability, whatever the cause, produce local distortions very similar to those produced by unequally heated air currents as just explained. As a result, the image of the chimney received upon the retina of the eye will be distorted though the chimney itself is perfectly straight. This phenomenon is apparent in much weaker light than is the distortion produced by convectional currents because the refractive index of glass is very much greater than that of air. If the uneven spot in the glass is equally transparent with the rest of the glass, it will be invisible unless some object (such as the chimney) is used for demonstrating the variation in refraction. This can be proved by viewing a uniform patch of blue sky through the glass defect, no distortion being evident under this condition. Density differences are only evident when an object of known form is placed behind the defect so that differences in its form and outline can be observed. Since a sound wave consists of alternate compressions and expansions of the air, density differences exist under these conditions just as with unequally heated air; variations in refraction also take place although with such rapidity that they cannot be observed with the eye unless assisted by special viewing devices. Likewise, density difference due to vapors and gases cause refractive differences if their density varies from that of the air, even though the vapors are perfectly clear and transparent under uniform conditions. These uneven areas in a medium (glass, air, etc.) are referred to as striae or streaks. This discussion of direct vision phenomena naturally brings up the question: "Can these so-called streaks and density variations in a medium be photographed and permanently recorded?" The answer is "yes," and further discussion will be confined to the photography of the invisible as applied in practice. The method of making these streaks | ||