ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 465 



edge of the cover-glass may be scraped off with a knife, and any 

 which still adheres to the glass may be removed with a little spirits 

 of wine. 



Small labels should be kept ready for affixing to the slides to mark 

 the locality and reference number of the specimen. Thus labelled the 

 slide may be put away for future study and comparison. 



The whole process seems, perhaps, a little tedious ; but in reality 

 much of it is so mechanical, that after the mode of manipulation has 

 been learnt by a little experience, the rubbing down may be done 

 while the operator is reading. Thus in the evening, when enjoying a 

 pleasant book after his day in the field, he may at the same time with 

 some practice rub down his rock-chips, and thus get over the drudgery 

 of the operation almost unconsciously. 



One final remark may here be required. The learner must not 

 suppose that, having prepared his slices, he has nothing to do but to 

 place them under the Microscope, and at once determine the composi- 

 tion. He will find it by no means an easy task to make satisfactory 

 progress, and at first he may be inclined to abandon microscopic work 

 in despair of ever gaining confidence in it. Let him, however, begin 

 by studying individual minerals, and make himself acquainted gradu- 

 ally with their various characters. He should procure numerous 

 sections of minerals which enter into the composition of the rocks 

 which he wishes to investigate. By degrees he will be able to dis- 

 criminate them as they occur in the rocks, and once able to do this, 

 his progress will be comparatively smooth. But he must be prepared 

 for a long, patient course of training, and ought on no account to 

 speak confidently about the microscopic structure of rocks until he 

 feels assured that the confidence arises from sound knowledge." 



Under the head of " The Microscope " the author explains the re- 

 quirements of the field-geologist to be 1^ in., 1 in., and ^ in. objectives, 

 giving powers from 30 to 300. It is always desirable to observe the 

 characters of a rock as an opaque object ; titaniferous iron, for example, 

 appears by transmitted light in black structureless grains or opaque 

 patches, whilst with reflected light the cleavage and lines of growth 

 of the mineral can often be clearly seen, and what seemed to be 

 uniform black patches are then found in many cases to inclose bright 

 brassy kernels of pyrite. With transmitted light somewhat different 

 appearances will be presented by two slices of the same rock, accord- 

 ing to the thinness of the section, brown or almost black minerals 

 appearing pale yellow, green, or almost colourless, when thinner. 

 Dichroism and polarized light are also dealt with, and the author 

 concludes with six questions which the student is to propound to 

 himself for his satisfaction in the determination of rocks. 



In his directions for preparing sections of fossil plants,* Dr. H. 

 Conwentz describes two grinding and polishing machines made by 

 Voigt and Hochgesang of Gottingen. 



The first (fig. 88) is a hand-machine. The specimen is cemented 



* In Behrens' ' Hilfsbuch zur Ausfiihrung mikroskopischer Untereuchungen 

 im Botanischen Laboratorium,' 1883 (Schwetschke u. Sohn, BrauDachweig), pp. 

 162-73 (5 figs.). 



Ser. 2.— Vol. III. 2 H 



