Herberger/Simon, Wissenschaftstheorie [1980], S. 243 [1]; M. Black = G. Frege, On sense and reference, in: P. Geach, M. Black (Eds.), Translations from the Philosophical Writings of Gottlob Frege, Blackwell Scientific Publications, Oxford, 1970, pp. 56--78.
The mentioned articles prepared Frege's: The Thought (Der Gedanke): A Logical Investigation. Beiträge zur Philosophie des deutschen Idealismus, I (1918), 58-77.