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V.—Notes on the Ash-Slates and Other Rocks of the Lake District

Published online by Cambridge University Press:  01 May 2009

W. Maynard Hutchings
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Extract

Whilst studying the sedimentary roofing-slates of North Wales and Cornwall, and allied materials, my attention was also directed to those most interesting rocks, the ash-slates of the Lake District, often externally so closely resembling some of the Welsh and Cornish examples, though differing so much in origin. In course of time I have collected, and had sections prepared from, a considerable number of specimens from many quarries and other places at various parts of the district, both very fine-grained wellcleaved actual roofing-slates and also the attendant coarser beds.

Type
Original Articles
Information
Geological Magazine , Volume 9 , Issue 4 , April 1892 , pp. 154 - 161
Copyright
Copyright © Cambridge University Press 1892

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References

1 In the matter of illumination, I have for a long time given up attempting to work by daylight, which in this climate is usually of too inferior a quality and too capricious for anything but low-power work. After experimenting in various ways I now make use of an ordinary good microscope-lamp burning paraffin. The lampchimney is of pale-blue glass, and the bull's-eye condenser used with it is also blued, by cementing to the flat side of it, with Canada-balsara, sufficient thicknesses of pale-blue glass so that the field of light obtained in the microscope appears bright white, like the best daylight. The adjustment of the quality of the light can also be controlled by observing the colours in polarized light of several points on a quartz-wedge (or better, on a mica-wedge made as recommended by Mr. Dick) first in good daylight and then in the light of the lamp. The depth of blue glass added to the condenser should be such that the colours are exactly the same in both cases. Then, with a good full flame and using the broad side of it, the light should be so thrown on to the mirror, from the flat side of the condenser, that the mirror is just covered with light. The effect in the microscope is the same as working with an exceptionally fine sky and one is sure that one's work will not be spoilt by a change in the light. Anybody who has once got used to working in this way and to the great advantage of always working with the same light will not again forego its advantages, but will, even in the daytime, make exclusive use of artifical light. My own experience also is that work with a good steady white light of this sort is less trying to the eyes than anything else.