NOTE ON THE SPECTRUM OF P CYGNI.

By A. BELO POLSKY.

I HAVE very recently obtained several spectrograms of the well-known star P Cygni (mag. 5.0) with our two-prism spectrograph, to which a camera of only 250 mm focus was attached. The spectrum extends from F to Hy, and the plates show the comparison lines of H, Fe, Sn, and N (air lines). As is well known, the lines in the spectrum of this star are both dark and bright; this is particularly the case on my plates for the He line at λ 4472 and for the hydrogen lines. The bright lines are about in coincidence with the artificial lines, while the dark lines are sharp (particularly λ 4472) and strongly displaced toward the violet end of the spectrum.

But my attention was especially struck by the fact that a dark and bright line correspond in the star's spectrum to each of the comparison air lines of the group from X 460 μμ to 465 μμ, with possibly one exception. The bright lines are faint and not measurable, but they nearly coincide in position with the artificial lines. The dark lines are displaced toward the violet. I give in the following table the wave-lengths reduced by Hartmann's formula of the lines measured on the plate of September 21 and 22, 1899. Column B gives the bright lines, D the dark lines, C the artificial comparison lines, and S the wave-lengths, according to Rowland, Runge, and Neovius.1

A repetition of the measurements of the differences between the comparison lines and the dark lines of the group 4607 to 4651 gave these results:

"Om skiljandet af Kväpvets och syvets linier." Stockholm, 1891.

The corresponding differences for the hydrogen lines appear somewhat greater: for F3.4, and for Hy 2.7 tenth-meters. This is certainly to be explained by silver precipitation of the bright lines overlying the dark lines at their edges, so that the observer makes his settings on the edge of the bright line, and not on the dark line.

No noteworthy changes from the above plate occur on the seven spectrograms of the star which I obtained between September 21 and October 1.

PULKOWA,

October 1899.

APPARATUS AND METHOD FOR THE PHOTOGRAPHIC MEASUREMENT OF THE BRIGHTNESS OF SUR

FACES.1

By J. HARTMANN.

On a photographic plate which has been properly exposed and developed, the blackening of the different portions of the picture corresponds directly to the photographic brightness of those points of the object photographed (the phenomenon of solarization being left out of account), and we may therefore conclude as to the brightness of the object from the degree of blackening. In order to carry out such photographic determinations of brightness in a precise manner the following points must be considered. We shall here always understand the brightness of an object to be the sum of those rays of light effective on the plate employed, which, after leaving the object and passing through the intervening media, actually reach the sensitive film. In each case it will be necessary to determine by special investigation to what extent the rays emitted by the object, for which the plate in question is sensitive, are lost by absorption in the air, as well as in the lenses and other optical parts of the photographic apparatus, which will depend upon the arrangement of the experiment. This definition of the photographic brightness corresponds exactly to that of the optical brightness as measured with ordinary photometers. For also in the case of optical measurements the intensity is determined of only those rays which actually pass from the source to the eye of the observer, and for which this eye is also sensitive. We should therefore expect differences in the determination of intensity between the eyes of different observers, as well as between different kinds of plates, particularly in the case of differently colored surfaces, and only measurements with the spectral photometer, whether they are made visually or photographically, are independent of this subjective definition of brightness.

1 Zeitschrift für Instrumentenkunde, 19, April 1899.

The first difference in principle between the photographic and the optical method arises from the property of the photographic plate that its blackening does not depend solely upon the intensity of the light falling upon it, but also in an equal degree upon the duration of exposure, upon the sensitiveness of the film, and upon the mode of development. It is true that within certain limits the correctness of the Bunsen-Roscoe law, according to which the blackening is proportional to the product of the intensity and the duration of exposure, has been also proven for bromide of silver gelatine plates. A law true only for special cases, the validity of which must first be proven by photometric measurements, cannot, however, be made the basis of such measurements, and for this reason photometric methods should be discarded which employ the different exposures of different parts of the photographic plate as a measure of brightness. For the determination of light intensities, use should not be made of rotating disks, with sectors of different sizes cut out of them, which are very suitable for sensitometer comparisons, in which the question is the determination of those exposures which produce equal blackening on different plates.

The only principle which can be made the basis of a photographic-photometric method, free from unnecessary assumptions, is this: Two sources of light are photographically equally bright when they produce equal blackening on one and the same plate with equal exposures.

We assume here only that every plate has the same sensitiveness over its whole surface, and that the development, and other treatment of the plate, have been precisely the same for all different points. If we should not make these two assumptions, the photometric utilization of photographic plates would be entirely impossible.

If any source L is to be compared with a standard lamp N, a portion of the plate is illuminated by L for a definite time from an accurately measured distance; at a neighboring point on the plate the scale is produced, the separate fields of which are obtained by precisely the same exposure at different distances

from the standard lamp. It therefore only remains to determine to which field of the scale the blackening produced by the source L corresponds, in order to calculate accurately, according to the fundamental law of photometry, the brightness of L expressed in units of the standard lamp. In exactly the same way the separate parts of a picture taken with any kind of photographic apparatus can be measured photometrically by a normal scale produced on the same plate, but it is to be remarked here that the brightness of the picture thus found is not to be immediately considered as the brightness of the object, as mentioned. above.

The process here given makes it possible to measure photometrically whatever can be obtained photographically; in particlar, objects can be rapidly measured in this way which have hitherto escaped direct photometric observation on account of their faintness, their rapid changes, or other reasons. I will only recall how laborious any accurate comparison of the brightness of the different formations of the Moon's surface has been hitherto, on account of the constantly changing illumination, while on a photograph of the Moon, which can be obtained in a second, the instantaneous distribution of light can be preserved and afterwards measured with great accuracy, at as many points on the image as may be desired. Similarly the brightness of comets. and nebulae is easily determined in this way.

The observation proper in this photometric method lies in the comparison of the picture with the fields of the scale which are found on the same plate. In sensitometer measurements such a comparison has to be made, but then the two exposed places are on two different plates. Hitherto this observation has com monly been obtained by placing the scale near that portion of the plate where the blackening is to be measured (by cutting the plate if necessary), and then estimating to which step of the scale the blackening corresponds. This process, however, is neither very reliable nor always applicable. If the place which is to be measured lies between regions of different degrees of blackening, or if the plate cannot be cut, then the scale cannot