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Parr IL. § vii] DETERMINATION OF ROCKS. 187 
forceps, we allow it gradually to rest upon the slice by letting 
down first one side, and then by degrees the whole. A few gentle 
circular movements of the cover-glass with the point of the needle 
or forceps may be needed to ensure the total disappearance of air- 
- bubbles. When these do not appear, and when, as before, we find 
that the balsam has acquired the proper degree of consistence, the 
slide containing the slice is removed, and placed on the table with a 
small lead weight above it in the same way asalready described. On 
becoming quite cold and hard the superabundant balsam round the 
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 localities and reference numbers. ‘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. 
Boxes with grooved sides for carrying microscopic slides are sold 
in different sizes. Such boxes are most convenient for a travelling 
equipage, as they go into small space, and with the help of a little 
_ eotton-wool they hold the glass-slides firmly without risk of breakage. 
For a final resting-place, a case with shallow trays or drawers in 
which the slides can lie flat is most convenient. 
2. The Microscope.— Unless the observer proposes to enter into 
great detail in the investigation of the minuter parts of rock structure, 
he does not require to procure a large and expensive instrument. 
For most geological purposes objectives of 13, 1, and 3 inch focal 
length with magnifying powers of from 30 to 70 diameters, are 
sufiicient. But it is desirable also for special work, such as the 
investigation of crystallites and inclusions of minerals, to have an 
objective capable of magnifying up to 200 or 300 diameters. An 
_ instrument with fairly good glasses of these powers, according to the 
arrangement of object-glasses and eye-pieces, may be had of some 
London makers for £5. But for some of the most important parts 
of the microscopical study of rocks a rotating stage is requisite, the 
presence of which necessarily adds to the cost of the instrument. 
One of the best microscopes specially adapted for lithological research 
is that devised by Professor Rosenbusch, of which an English modi- 
- fication is made by Watson of Pall Mall, London, and sold at £21. 
It contains every apparatus required for ordinary work. A less 
complete but useful instrument is sold by the same maker 
for £9. 10s. 
Among the indispensable adjuncts are two Nicol prisms, one to be 
" fitted below the stage, the other most advantageously placed over 
