A possible influence of comets on the growth of heavy elements abundance suggested several years ago by Tinsley and Cameron is shortly reviewed. Such an effect would be significant if the formation of comets produce cometary material equivalent to 0.01 - 0.02 M per one solar mass. In the view of current hypothesis of the formation and structure of cometary nuclei the role of comets should be considered as one of the possible moderators of the Z value variation during the chemical evolution of our Galaxy.
The present paper is an evaluation of photoelectric observations of teh eomts 1956h amd 1957d, made at the observatory of the University in Brno. The observations were carried out both in integral light and in selected spectral ranges, corresponding to the international colour system. the colour index of the comet 1956h indicates a sudden drop on May 2nd, which has been verified also by other observations. This drop is explicable only by an increased emission of gases into the coma. The mean colour index is 0.58, which corresponds to the colour index of stars of the spectral class GO. The mean colour index of part of the tail is - if we disregard the drop on May 2nd - +0·78. The photometric parameters, derived from flawless observations are m0υ = 6·55. nυ = 2·03 and m0B = 7·15, nB = 2·30. One good observation of the comet 1957d yielded the colout index of 0·15.
The authors attempt to derive some fundamental properties of the atmospheres of comets from collected photometrical quantities, primarily of classical photometric parameters.
In the first part various, currently used methods of measurements are being evalulated and some modifications or applications of them suggested. The authors point at the main insufficiencies of the methods, consisting, first of all, in the fact that the majority of them canot be cosidered as measurements in the proper meaning of that word.
In the second part the physical significance of the photometrical parameters is being theoretically interpreted. On the basis of actual material it was found that the gas-dust model of the atmospheres of comets is an acceptable explanation of the dependence of photometrical parameters on the heliocentric distance.
In the last part it has been briely demonstrated that in some cases, particularly in those of short-period comets, the alteration of the photometrical parameters with time need not be caused by change in the percentage of the meteoric dust, dispersed in the coma. The total amount of dust at the moment of observation is in comets with a continuous spectrum of the order of 10^11 g.
This paper presents measurements of the polarization of light in the tails of the comets Arend-Roland and Mrkos, made in the total photographic light. Results given in Tables II and V indicate that the tail of the Arend-Roland comet contained a substantial dust component, while the polarization in the tail of the Mrkos comet can be explained predominantly by pure molecular emission.