Fewster Paul F
Brighton, UK.
Acta Crystallogr A Found Adv. 2018 Sep 1;74(Pt 5):457-465. doi: 10.1107/S2053273318007489. Epub 2018 Jul 18.
The criticisms of my theory, as given by Fraser & Wark [(2018), Acta Cryst. A74, 447-456], are built on a misunderstanding of the concept and the methodology I have used. The assumption they have made rules out my description from which they conclude that my theory is proved to be wrong. They assume that I have misunderstood the diffraction associated with the shape of a crystal and my calculation is only relevant to a parallelepiped and even that I have got wrong. It only appears wrong to Fraser & Wark because the effect I predict has nothing to do with the crystal shape. The effect though can be measured as well as the crystal shape effects. This response describes my reasoning behind the theory, how it can be related to the Ewald sphere construction, and the build-up of the full diffraction pattern from all the scatterers in a stack of planes. It is the latter point that makes the Fraser & Wark analysis incomplete. The description given in this article describes my approach much more precisely with reference to the Ewald sphere construction. Several experiments are described that directly measure the predictions of the new theory, which are explained with reference to the Ewald sphere description. In its simplest terms the new theory can be considered as giving a thickness to the Ewald sphere surface, whereas in the conventional theory it has no thickness. Any thickness immediately informs us that the scattering from a peak at the Bragg angle does not have to be in the Bragg condition to be observed. I believe the conventional theory is a very good approximation, but as soon as it is tested with careful experiments it is shown to be incomplete. The new theory puts forward the idea that there is persistent intensity at the Bragg scattering angle outside the Bragg condition. This intensity is weak (∼10) but can be observed in careful laboratory experiments, despite being on the limit of observation, yet it has a profound impact on how we should interpret diffraction patterns.
弗雷泽和沃克[（2018年），《晶体学报》A74卷，447 - 456页]对我的理论的批评，是基于对我所使用的概念和方法的误解。他们所做的假设排除了我的描述，据此他们得出我的理论被证明是错误的结论。他们假定我误解了与晶体形状相关的衍射，并且我的计算只与平行六面体有关，甚至认为我算错了。在弗雷泽和沃克看来它似乎是错的，只是因为我所预测的效应与晶体形状无关。不过这个效应和晶体形状效应一样是可以测量的。本回应阐述了我理论背后的推理过程、它与埃瓦尔德球构造的关联，以及从平面堆叠中的所有散射体形成完整衍射图样的过程。正是后一点使得弗雷泽和沃克的分析不完整。本文给出的描述更精确地参照埃瓦尔德球构造阐述了我的方法。文中描述了几个直接测量新理论预测结果的实验，并参照埃瓦尔德球描述对其进行了解释。简单来说，新理论可以被认为是赋予了埃瓦尔德球表面一个厚度，而在传统理论中它没有厚度。任何厚度都立刻告诉我们，在布拉格角处来自一个峰的散射不必处于布拉格条件下就能被观测到。我认为传统理论是一个非常好的近似，但一旦用精细的实验进行检验，就会发现它是不完整的。新理论提出了这样一个观点，即在布拉格条件之外的布拉格散射角处存在持续的强度。这个强度很弱（约为10），但尽管处于观测极限，仍能在精细的实验室实验中被观测到，然而它对我们解释衍射图样的方式有着深远的影响。
