760 HANDBOOK OF PHOTOGRAPHY 



the sun which is visible only when the brilliant disk of the sun itself is hidden by the 

 dark disk of the moon that completely covers it during the moments of total eclipse. 

 A total eclipse of the sun may last from only a few seconds to a period of slightly less 

 than 8 min. Exposures on the corona are usually made with lenses of great focal 

 length to obtain images of requisite size to show the desired details. Suitable exposure 

 times will vary from 1 sec. to 3^2 min. or more. Usually a series of plates will be taken 

 with varying exposures. The short exposures are necessarj^ to gain detail of the struc- 

 ture of the corona near the edge of the sun. The longer exposures will reveal the out- 

 lying stretches or extremities of the corona, but the inner parts on the resulting 

 negative will be greatly overexposed. By suitable local reduction of the negative, 

 however, it is often possible to get a fair representation of the entire structure of the 

 corona from the edge of the sun to the outlying rims. A plate locally reduced, how- 

 ever, loses much of its scientific value since the relative blackening of the various 

 parts of the corona in the untouched negative provide an index of the relative bright- 

 ness of the emission of the light from the various regions. In preparing the plates for 

 eclipse photography, it is quite necessary that the plate be backed with any one of a 

 number of suitable backings to prevent halation, or double-coated nonhalation plates 

 should be employed. 



Lenses up to 40-ft. focal length are frequently transported to all parts of the world 

 on eclipse expeditions. Since it is possible to calculate in advance the exact place 

 which the sun will occupy in the sky during the moments of eclipse, temporary struc- 

 tures can be built for supporting such lenses and the elaborate equatorial mounting of 

 the telescope dispensed wdth. In the case of the cameras of great focal length, usually 

 no attempt is made to move the lens during the period of exposures, but the plate- 

 holder itself is mounted on a carriage, and clockwork is provided for moving the same 

 laterally at right angles to the optical axis in order to follow the small shift in the 

 image of the sun during the exposure, thus facilitating very greatly the mechanical 

 arrangements involved. In the case of a total solar eclipse lasting 4 min., the move- 

 ment of the solar image during this interval for a lens of 40-ft. focus will be approxi- 

 mately 8.4 in. This gives an idea of the amount of moA^ement of the plate required 

 for an eclipse of average duration. The necessity for providing motions of the plate 

 during even a single exposure becomes apparent when we see that in this instance the 

 solar image would be displaced by 3^^ in. during even the relatively brief exposure of 

 15 sec. Photographs of eclipses with ordinary' cameras having no guiding mechanism 

 have been satisfactorily made by giving very brief exposures in cases where the lenses 

 used are of but a few inches focal length. Such photographs, however, can scarcely 

 be said to have much professional or scientific value. 



The application of motion-picture photographj!" to the registration of eclipse 

 phenomena, however, has come to occupy an important part in connection with 

 eclipse expeditions. With the best of lenses of 20-in. focal length, an image of the sun 

 of satisfactory size can be secured on the ordinarj^ 35-mm. film. The complete regis- 

 tration of both the total and partial phases of the eclipse forms an important record of 

 sky conditions, transparency, passing of clouds, etc. It is often of considerable value 

 afterward in checking visual impressions. During the partial phase it is necessary 

 that the smallest stop possible be employed, together with a dense red filter, in order 

 to diminish sufficiently the direct rays of the sun to prevent overexposure. At the 

 moment of complete obscuration of the solar disk by the encroaching moon, the filter 

 must be removed, and the stop opened widely in order to obtain the requisite amount 

 of exposure for the corona itself. At the first indication of the reappearance of the 

 disk of the sun, the aperture is again reduced and the filter restored while the motion- 

 picture camera continues to record the second partial phase. 



