lines, however, a constant value may be used to obtain an approximate correction, which was applied in the following manner 5): the zero of the intensity scale was not placed at the actual zero recorded by the microphotometer, but at the point where the intensity is 3 per cent (or 0.75 per cent) of that in the continuous spectrum. This was accomplished by using a maximum deflection of 103 mm in recording the second-order Mount Wilson plates, after which the zero line of the intensity scale was printed 3 mm above the recorded zero line. This direct procedure could be applied here only because the ordinate on the tracings is proportional to the intensity of the radiation in the spectrum; ordinary microphotometer tracings ccmnot be corrected for the effect of ghosts in so simple a way.
Moll microphotometer *). In the microphotometer the lamp throws a narrow beam of light through the plate upon the slit of a thermoelement. The thermoelectric current is now sent to an auxiliary galvanometer, Gl, illuminated by a long luminous slit S. A small lens in front of the galvanometer mirror forms an image of this slit on a diaphragm, D, which, cut out of strong squared paper, has to be made anew for every spectrogram. The light transmitted is collected by condenser lens, A, on a photocell, C, which is connected with G,, the regular galvanometer of the microphotometer. Finally the deflection of G 2 is recorded on photographic paper moved by the drum R. The accuracy of the results depends largely on the correct shape of the diaphragm D. The characteristic curve (intensity plotted against deflection of galvanometer G 1) of the spectrogram to be measured serves as a first approximation; to this a small empirical correction has to be applied, eliminating the combined effects of the imperfections of the apparatus. Bef ore every run, the instrument was adjusted so as to make the deflection of G, proportional, within one per cent, to the intensity of the light, received by the photographic plate during exposure. A more detailed description of the instrument has been published elsewhere 7). Continuous Spectrum. The deflection corresponding to the continuous spectrum was always kept as nearly as possible at a standard value of 100 mm (not including the few millimeters for ghost-correction). On many plates the density of the continuous spectrum gradually changes along the spectrum. This may be due to several causes, for example the variation of the sensitivity of the emulsion with wave length. We compensated for such gradients by moving a wedge synchronously with the plate through the light-beam LiS by means of a lever. In this way the maximum deflection on the tracing could be kept approximately constant, although a slight curvature of the continuous background may still be present. A list of the regions where the compensating wedge had to be used (Table III) may be helpful in discussing the exact position of the continuous spectrum.
The Ultraviolet Spectrograms (2. 3332-2 3637).
For wave lengths shorter than A 3600 the solar spectrum could not be photographed through the glass optics of the 150-foot tower telescope at Mount Wilson. Hence it was decided to photograph as much of this region as possible with an improvised arrangement at the Physical Laboratory at Utrecht. The glass objective of the Utrecht solar telescope 6) was removed. A plane mirror near the lower end of the telescope deflected the light horizontally towards a concave mirror, projecting a solar image on the grating of the improvised spectrograph. A Wood concave grating 13 by 8 cm in size, hoving 591 lines to the mm and a radius of curvature of 10 m, was used. The spectrum of integrated sunlight was photographed in the second order, where the dispersion is about 1.2 mm/A. Because of the astigmatism, inherent to a concave grating used in Rowland mounting, the quartz step weakener used for the standardization had to be placed at a certain distance in front of the slit; this distance varied with wave length.
When the sun was shining on the black curtains bef ore the windows of the room in which the grating was set up, air currents caused bad seeing which impaired the definition of the spectrum. This poor seeing, the long exposure times, and the small number of sunny days forced us to use the very sensitive Ilf ord Double X-Press plates which, unfortunately, have a coarse grain. Nevertheless, usable spectrograms were obtained thanks to the persistence of Mr. J. H. Bannier who photographed this region.
TABLE 111. REGIONS RECORDED WITH A COMPENSATING WEDGE Appendix 2nd order lst Order 2k.A 3332-3401 3612-3663 4503-4520 5773-5854 6924-6973 3414-3519 3697-3714 4569-4586 5965-5982 7074-7126 3558 3575 3762-3779 4603-4619 6085-6094 7262-7382 3591-3637 3828-3877 4685 4702 6152-6200 7416-7635 3892-3909 4767-4776 6216-6233 7669-7870 4000-4065 4832-4849 6282-6314 7887-7938 4090-4123 4864-4931 6378-6395 8020-8055 4156-4205 4952-5243 6539-6555 8071-8405 4222-4273 5323-5339 6577-6593 8422-8473 4288-4305 5404 5606 6603-6697 8518-8602 4370-4387 5654 5683 6712 6831 8620-8654 4420-4437 5693-5757 6839-6979 8670-8771 *) In this investigation, as in many previous ones, the great reliability and practical design of the. Moll microphotometer were confirmed again and again.
The total intensity of the ghosts which the concave grating produces in the second order is about 7 per cent of that of the principal line. An approximate correction for this effect has been introduced in the way described in the preceding section. In the ultraviolet the numerous absorption lines reduce the average intensity of the background to perhaps 70 per cent of that of the actual continuous spectrum. For this reason the zero of the intensity scale on the tracings was placed 5 mm (7 per cent of 70 mm) instead of 7 mm above the recorded zero line.
The results are presented in an appendix. They are preliminary, but af ford the only photometric data now available for this region of the solar spectrum.
THE METHOD OF RECORDING INTENSITIES
The usual process of trcmsforming microphotometer trctcings into intensity curves would have been too laborious, in view of the large region of the spectrum to be covered. Hence an instrument was designed to record intensity curves directly. All the tracings published here were registered by this instrument. Attached to any ordinary microphotometer it can be highly recommended for large-scale photometric work.
Figure 2 shows a diagram of the instrument used in combination with the well-known