D O C U M E N T 1 8 2 M AY 1 9 2 8 1 8 1 182. To Edward H. Synge [Berlin,] 3 May 1928 Dear Sir, First of all, I would like to express my pleasure at the fact that the publication of Hamilton’s life’s work has been undertaken.[1] This will indeed be of real service to the scientific world.— Let me now proceed to your scientific ideas. In itself, the following basic train of thought seems correct to me: If I have a light source L with a dimension of 10-6 cm and move a layer S past it at a close distance, then by observation of the fluctu- ations in the amount of transmitted light, I can reach conclusions about the struc- ture of the immediately neighboring portion L of the layer.[2]— Up to this point, I am in agreement. However, your method of carrying out the experiment appears to me to be impracticable in principle. If, namely, the distance of the layer S from your lower quartz platelet is small compared to the wavelength of the light used, then the layer S will itself become a light source, so that the entire field will become bright.[3] If, however, the distance of the layer S from the lower quartz platelet and thus from the light source L is greater than the wavelength of the light, then the attainable scale of the structural information about the layer will not essentially exceed the resolution available with a usual microscope.— In principle, this difficulty can be overcome, for example by: A. The exciting light is outside the visible region and excites the colloidal parti- cles, but not the layer S itself, to emission.[4] B. As a light source, the light that passes through a very small opening in an opaque screen is used. A genuinely practically applicable method of implementing your nice idea has not occurred to me.— Perhaps I have failed to understand it correctly, but I was unable to read so much in your letter, that is, I had to guess at it, so that it is indeed possible that your letter nevertheless already contains the solution to the problem.[5] I would be happy to hear your opinion about this.— Respectfully yours,