How cite sources of 100-years-old analytical technique in a theory section?
The real question is whether you need to cite anything at all.  In your field, if this information is considered undergraduate-level material that every practitioner should be familiar with, then you can simply assume that all readers will be able to reference their favorite textbook if they care, and launch right into the relevant review.

If you are speaking to an audience that may not be so familiar, however, you definitely do need to cite.  You don't need to put in lots of citations, however: you can simply say that this is well-established information and give one citation to a good textbook:


  The principles of X-ray diffraction are well-known (see, e.g., [textbook]).


For example, when I am writing to a computer science audience and need to say something about the NP-completeness of constraint satisfaction, I would probably not include a citation, since this is such a foundational concept to the field at this point.  When I write to a biology audience about the same problem, however, I explicitly note that these are long-established facts and include citations to basic textbooks.