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Eclipse Bulletin No 3 cont'd. 4. TIMING THE FLASH SPECTRUM For a brief instant at the beginning and end of totality, the narrow dark Fraunhofer absorption lines of the sun's spectrum are reversed, that is, they shine out as brighb lines, the more intense backgxvund of the light from the surface of the sun having been cut off. If binoculars are used to obsene the flash spectrum, a grating (4000 lines or more to the inch) should be placed over one objective and welder's glass over tkt other. The observer can then watch the approach of totality through one lens and be ready to observe the flash spctrum through the other. A telescope can be used with either a grating over the objective or a spectroscope attachment at. the eyepiece while the finder, with welder's glass over the dbjebtive, can be used to watch the approach of totality The flash spectrum should be timed as accurately as possible and the observer's impressions as to the, intensity of the lines, etc. should be recorded. A clock drive on the telescope would be an advantage. 5. TIMING THE SHADOW BANDS The shadow bands are fleeting shadows that race along the ground before ans after the main shadow. The direction of these bands and their line of motion differ before and after totality and with the position of the observer relative to the,line of totality. They can be best seen against a white background. Therefore, two white sheets should be spread on level ground- one to observe the bands before totality and the other for after totality. A team of three is required - one timekeeper and two observers. The procession of shadow bands may last from ten to fifteen, seconds. The observers call "Time!" the instant the bands appear and again when they disapp- ear, the timekeeper recording the times to the nearest second. One observer has a supply of sticks or bamboo rods which, as the bands appear, he quickly places on the sheet to indicate their dir ection and line of motion. The other observer con- centrates on the width of the bands, whether they are straight or wavy, their colOur and any other characteristics, The rods `are left in position so that, with the aid of a compass, their true direction can be determined unhurriedly after totality is over. All such details as to time, duratIon, direction, will be of value from every point on the eclipse path. It will be seen that for this project the team will need to practice to get operations down to a routine. 6. METEOROLOGICAL DATA There are two reasons for recording weather' conditions - (1) because of the. effect of'the weather on observations made, and (2) because of the effect of the' eclipse on atmospheric conditions. For the former, one needs no special eqiupment, simply recording the observing conditions (seeing, transparancy, cloud cover) during the period of the eclipse. These should be recorded at five-minute intervals and; in addition, whenever a change in conditions occurs. For the latter, Equipment con- sisting of wet and dry thermometers, barometer, anometer, photometer, etc., should be in operation for a fUll hour before first contact and a full hour again after fourth contact. During these tqo one-hour periods, readings should be made at five- minute intervals. During the eclipse - the partial phases and totality readings should be made at two-minute intervals. An electric clock, with sweep second hand, would be sufficiently accurate for recording the readings. If possible, the equip- ment should be installed on the previous day and the team should make readings for the same period as on eclipse day. Not only will this furnish comparison graphs but will give the team the practice needed to carry out operations smoothly on eclipse day. - 4 -