The well-known, frequently observed phenomenon of coloration of twinned plates that grow through calcite grains along the planes of the first most obtuse rhombohedron (1012) also belongs to the category of pseudochromism phenomena. In this case, the pseudochromism phenomenon is often very striking, and for this reason, as well as because the position of the twinned plates is here easily determined, the phenomenon can be more readily subjected to investigation. This work represents an attempt at such an investigation. The application of the "universal" method to the study greatly supplements the picture of the phenomenon and makes it possible to answer some of the questions it raises. Therefore, I venture to publish the obtained results despite being aware of the incompleteness in the elucidation of the issue.

The following issues are considered in the work: 1. Determination of the magnitude of birefringence from readings taken with the compensator and the investigated grain in direct and reversed parallel positions. 2. The degree of sensitivity of the rotating compensator, the Siedentopf compensator and the Fedorov quartz and mica compensators.

In one of the thin sections of quartz porphyry from the Vtoro-Blagodatny mine, I came across an interesting case of fractures within feldspar grains being filled with feldspathic substance of the same composition and in the same orientation. The rock is quite strongly altered: the feldspars are abundantly strewn with muscovite dust. The thin section field is cut by several veins, apparently filled with new growths. In this case, depending on whether the vein cuts through the groundmass or the feldspar segregations, it is filled either with an aggregate of feldspars and quartz or with feldspars alone, with such a slight difference in orientation compared to the mineral of the grain cut through by the vein that it cannot be quantified. Feldspar veins are characterized by significantly greater transparency due to the absence of muscovite inclusions.

The purpose of this short note is to present several new devices and techniques that I introduced during my work on the universal optical (in common speech Fedorov's) method of microscopic research and which have been already recognized by persons who use this method in their work: 1) segments in a metal frame; 2) rotating comparator; 3) measuring the thickness of a thin section using cracks and inclusions in it by tilting the specimen; 4) hemisphere for graphic solutions to problems of the universal optical research method.