继续看关于转基因方面的论文，在看到卵母细胞如何去细胞核的时候，有两种方法：盲吸法以及借助显微镜法。其中盲吸法不可避免的造成去核不彻底（可以通过离心3000r/min来解决此问题—《Vol.32.No1.2010Chinese journal of Cell Biology》），亦可通过借助显微镜法，包括两种：霍夫曼镜头以及微分干涉镜头。可以清晰的显示其细胞核，但是要用于转基因显微操作，霍夫曼镜头是首选，因为其工作距离较长，且可以利用塑料培养皿。
在网上找了很久，没有找到中文的关于霍夫曼原理的资料，结果英文的倒是一大堆，在这里罗列一些，如果网友有中文的资源，欢迎分享！如果有物理方面的学者帮忙翻译一下，就更好了！
HOFFMAN MODULATION CONTRAST (HMC) (Inventor: Robert Hoffman, 1975)
The Hoffman modulation contrast (HMC) microscope is another method by which transparent, or nearly so, objects can be visualized. Hoffman modulation contrast accentuates phase gradients within the sample and displays them in the image plane as levels of gray modulated lighter and darker than an average background gray. To accomplish this, a special filter is placed in the Fourier plane (back focal plane) of the objective conjugate to another filter, the condenser slit. The modulator filter is constructed so as to have three regions of different neutral densities, ranging from low to a high attenuation, usually T=100%, 15%, and 1% (Figure 2-6A). The condenser slit is adjusted so that light transmission through the slit falls on the gray region (15%) of the modulator.
The HMC microscope exploits phase gradients within the sample. In regions where there is a rapid spatial change of sample optical path, refraction will occur, slightly shifting the path refracted light.17 Light passing through negative amplitude gradients (light to dark) within the sample will be refracted through the dark zone of the modulator and be rendered darker. Light that passes through homogeneous areas within the sample will experience no refraction and pass through the central gray region of the modulator and be rendered uniformly 15% gray. Light passing through positive gradients (dark to light) will be refracted through the light zone and remain bright. Thus, light passing through the modulator is accentuated in contrast and results in an image with pseudo-relief (much like DIC) with the three-dimensionality representative of light phase gradients rather than actual object geometry. Unlike DIC, HMC uses no beam-splitting prisms. Furthermore, the two polarizing filters (P1, P2) are placed optically in front of the sample. Thus, HMC can be used with birefringent specimens not amenable to DIC (e.g., crystalline objects or specimens in plastic Petri plates).
