But having thus arranged, though the arrangement is but temporary, the specimens gathered, on slips of white paper, the next and most important point is their due examination and determination. This, in the present advanced state of Lichenology, is unquestionably, in many cases, a task of considerable difficulty, and in the short space at our disposal it would be quite impossible to give anything like an adequate explanation of the mode in which this is to be effected. Suffice it at present to say that sections must be made of the thallus to ascertain the character of its different layers, as also sections of the apothecia and spermagones to ascertain the nature of the spores and spermatia. For both purposes a good microscope, with 1/4-inch object-glass, is absolutely indispensable to the student. The examination of the spores, upon which, in so many cases, the determination of the species chiefly depends, should present little or no difficulty, at least to the fungologist. It may be readily effected by moistening the apothecium with water, and then, with a dissecting-knife, making a thin vertical section through its centre. Putting this on a glass slide, or in a compressorium, in a drop of hydrate of potash, and then placing it under the microscope, a view will be obtained of the asci, spores, paraphyses, hypothecium, &c., each of which may afterwards be insulated and examined more minutely in detail. Take, for example, the well-known beautiful yellow lichen (_Physcia parietina_), so common everywhere on walls, rocks, and trees, and treat a very thin section of the mature apothecium as before mentioned. Under the microscope it will appear as represented in Fig. 43. In the same way the spermagones may be examined, when the nature of the sterigmata and spermatia will be apparent. By cutting across the thallus of the above species, we can perceive even by the naked eye that it consists of three different layers, which when microscopically examined present the appearance shown in the above figure. [Illustration: Fig. 44. Section of _Physcia parietina_. _a._ Cortical stratum. _b._ Gonidic stratum. _c._ Medullary stratum.]