630610Eclipse7_p3

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Eclipse Bufletin No. 7 cont'd                                                -3-


    Probably the best filters to use are the Wratten 25 (red) and 47 (blue); (see
    Handbook of Physics and Chemistry). At least one short and one long exposure
    must be made through each filter during totality; a total of four or more, each
    lasting several seconds.   Although the image on the film must be of reasonable
    size, it is obvious that a rather "fast" optical system must be used.   A steady,
    clock-driven mounting is essential.   A medium-grain black-and-white film is
    probably the best to use.

    In submitting his results for analysis, the observer should enclose the actual
    filters used, as these are not perfectly uniform. For the purposes of calibra-
    tion, afl aposures must be made on a single roll of film, which is also to be
    used later to make test exposures for determining the film' s characteristics,
    notably the relations between wave-length aid sensitivitq, and between exposure
    time and, darkening. The lengths of the exposures must in all cases be known
    very accurately, aid a note on the estimated accuracy of measurement must be
    supplied.

6.  Photography of the Spectrum with PolarizedLgt: It has been found in the past
    that the light fran the corona is polarized. One observer intends to take
    photographs of the coronal spectrum through a polarizing prism, rotating the
    latter between exposures, to determine whether the polarization is the same for
    all the (emission) lines, or whether the effect varies for the lines of different
    elements.

    It is hoped that there will be enough time to make four exposures during totality,
    using a 6" f/10.5 reflecting telescope with a grating spectrograph as shown in the
    diagram. After the end of totality a number of comparison spectra will be made
    from the sun and from an incandescent bulb, in order to offset errors arising from
    interference fringes caused by a lacquer coating over the surface of the grating
    to be used.




    (Diagram here.)



7.  Recording Changes in the flash Spectrum: A well-timed succession of spectrograms
    showing the reversal of the lines at the onset of totality is at value. For the
    1959 eclipse three members of the Ottawa Centre devised the camera shown here. A
    description follws:

    "Our camera lens was an air-spaced achrcxnat of diameter 4 in. and focal length
    42 in. We used no slit, but mounted a large diffraction grating ..., in front
    of the lens.   (It) had about 5000 lines/inch and .... was blazed in the first