The absorption spectra of cadmium sulphide single crystals were observed with natural light and polarized light, one piece of the crystals having a thickness of about 0.1 mm and another about 0.05mm.At room and liquid air temperatures, the absorptions spectra observed with natural light indicate that the wavelength of the long wavelength limit of CdS depends upon the thickness and the temperature of the crystals.A change of the wavelength of the absorption edge which depended upon the orientation of the electric vector of the polarized light produced by a polaroid plate placed between the crystal and the slit of the spectrograph was observed. At room and liquid air temperatures, the absorption spectra of the thinner CdS crystal with polarized light were observed .The spectrum at the liquid air temperature was a hydrogen like series which consisted of seven members of strong and sharp lines located in the long wavelength side of the absorption edge of the crystal. From the results of the wavelength measurements we found that the wave number of each spectral line fulfils the following relation: where k= 3 , 4 , 5 ,...9, and the line of k=2 was not seen in the spetrum. From the above emperical formula the series limit of the hydrogen-like series of CdS crystal after the trmperature correction was found to be 2.41 eV , which is in good agreement with the value of 2.42 eV obtained by othe methods. The reduced mass of the exciton in CdS crystal was found to be about 4.3 times the mass of electron. CdS and ZnS crystals have a similar strueture, and the numerical values of their lattice constants are quite near. It could be infered that the full band of CdS crystal is pobably due to the S--ions and the observed hydrogen-like series of the excitons spectra is probably due to the excited state of S--ions.