PART I. An investigation of the gamma radiation from polonium: 1. The investigation of the absorption of the polonium gamma radiation in lead, gold and tungstan revealed that in addition to the established hard component of 0.8 Mev. energy, some softer radiation is emitted, of which the main component has the energy of 84±4 kev. 2. In the second part of the investigation the alpha disintegration product of polonium was tested for short lived gamma activity, by using the recoil method of separation. No positive effect could be detected. PART II. Further applications of the photographic emulsion track method: 1. An investigation of the electron tracks in the new electron sensitive NT²ᵃ and NT⁴ emulsions produced by Kodak Research Laboratories in England, has been carried out. The range-energy relations, and the relations between the total numbers of grains and grain densities along the track, and the residual ranges of the electrons of energies from 30 to 250 kevr in the NT⁴ plates have been established. 2. The NT⁴ plates are in general better of the two types. They are capable of registering tracks of particles at minimum ionisation, while within the energy range investigated they give good, clear electron tracks, easily observed under the microscope and consequently inducing less strain on the observer. The NT²ᵃ emulsions register tracks of electrons of energies up to some 50 - 80 kev. The examination of these plates is much more tedious. 3. The NT⁴ plates are suitable for measurements of tracks of electrons of energies down to about 30 kev. The corresponding limit for the NT²ᵃ plates is 25 kev. For investigations in this low energy region the NT²ᵃ plates seem more suitable. 4. No high resolution of energies can be attained attained by the use of the photographic plate method on account of the large straggling effects for the electrons in the emulsion. 5. The most severe limitation on the applications of the photographic plate method of investigation is imposed by the rapid accumulation of "track background" in the plates due to the action of cosmic radiations. No satisfactory method of removing this background has yet been devised. Without any special criteria for distinction between the "proper' and "background" tracks in a particular problem, the work would become unduly tedious and the results would have only a statistical significance. Successful application of the photographic plate method in the investigations of very weak/active substances seems at this stage quite impossible.