, 
Physics. — “Photographic Absorption- and Eatinction- Measurements. 
Contributions to the study of liquid crystals. V. Evtinction- 
\ 
measurements”. ) By Miss Rassa Riwrin. (Communicated by 
Prof. W. H. Junius). 
(Communicated at the meeting of May 29, 1920). 
In this paper a photographie method will be explained for measuring 
absorption-spectra, and a preliminary application thereof to the 
examination of the extinetion of fluid erystals. The purpose of this 
research is: to look more closely into the way in which the extinc- 
tion depends quantitatively upon the wavelength and especially to 
trace in what degree the difference, which Dr. W. J. H. Morr and 
Prof. Dr. L. S. ORNsteiN found between the phases ex-solid and 
ex-fluid ®) in the ultra-red, exists too for visible light. 
1. The extinction (absorption or dispersion) of a substance can be 
measured by the following method. A pencil of parallel rays pro- 
ceeding from a constant source of light runs through the object. 
After passing through the substance a spectrograph disperses the 
light into a spectrum and this spectrum is photographed. After this 
we remove the substance and substitute it successively by a few 
screens which reduce the incident light to a known degree; the spectra 
obtained in this way are photographed each time. It would be 
obvious to try and find for each colour the screen which causes on 
the photographic plate the same blackening as the preparation. The 
faculty of transmission of the screen being known, that of the pre- 
paration for the considered wavelength is equal to it. Practically, 
however, this method — the looking for places of equal blackening — 
is inconvenient and therefore an interpolation method is substituted 
for it. For every definite wavelength namely the blackening is 
found out of the different screen-spectra, which are marked as a 
function of the intensity of light. By means of the blackening-curve, 
in this way experimentally constructed, the blackening of the substance 
for every wavelength immediately indicates the desired faculty of 
') Cf. YNevre BJÖRNSTHAL, Untersuchungen über Anisotrope Flüssigkeiten. Ann. 
der Phys. Bd. 56 (1918), p. 161. 
*) Proceedings Vol. XX NO. 2 p. 210. 
