Click on the thumbnail at the bottom right of this page to go to the bitmap scan of this document. The text of this document appears immediately below.
Eclipse Bulletin No, 4 cont'd. 2. TOTALITY (b) Corona. colour. Here again it has been pointed out that the anateur can make significant contributions. "Observations have been made at previous eclipses which indicate that the colour of the inner corona is not the same as the outer corona but the results are discordant. The usual method is to take well calibrated photo- graphs of the corona using fflters of two colours (red and blue) and note intensity differences. The experiment, although simple in principle, is however a difficult one in practice since it is diffic4t to achieve the required accuracy of calibrat- ion and freedom from scattered light which may invalidate results." (c) Prominences. The prominences red, fiery tongues of light at the inner edge of the corona are sometimes very fine, sometimes missing entirely. This detail can be obtained only by photographing through telescope or binoculars. 3. SHADOW BANDS The phenomenon of the shadow bands is described in Bulletin No.3. They are very difficult to photograph on account of the low illumination and the speed of the movement. Use a short exposure time (about 1/100 second), a large aperture (f/2.0) and a high speed film (Tri-X). The camera should face downward toward a white sheet on which is placed a compass and some scale for measuring widths of the bands. As explained in Bulletin No. 3, they should be observed both before and after totality. Because of their short duration, the photographer has to be on Us alert if he hopes to take more than one exposure. 4. LANDSCAPE ILLUMINATION It is interesting to record changes in landscape tllumination by selecting a subject presenting a fair range of contrast, aiming the camera and taking photo- graphs at regular intervals throughout the eclipse, say, every five minutes with one exposure at mid-totality, using a constant exposure. Exposures made at the beginning and end will probably have to be somewhat over-exposed if anything is to register during totality. 5. SPECTROSCOPIC PHOTOGRAPHY (a) Flash Spectrum. The phenomenon of the flash spectrum is described in Bulletin No. 3. It is of very short duration (about 2 seconds), and so the photographer has little margin for error in taking his shots. The flash spectrum can be photo- graphed by placing a prism or diffraction grating over the lens of an ordinary camera and timing the camera so that the spectrum enters the lens. If a prism is used, the lens should have a long Local length (about 100 mm.). Black-and-white film should be used if measurements are intended; otberwise, colour film gives interesting results. The shutter of the camera should be set to B (bulb), the lens wide open and focus at infinity, and a cable release used, The observer, watching through another prism or grating, should press the release as soon as the bright-line spectrum appears, and release it as soon as possible after the onset of totality. Another method, whereby the entire change in the spectrum can be obtained by winding the film continuously, will be described in a later bulletin covering special projects. - 3 -