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POPULAR SCIENCE REVIEW. 
sable instrument of research in the study of physical geology 
and petrology, as well as indicated how much more may be 
expected from its more extended application. 
The application of the microscope in these enquiries is as yet, 
however, quite in its infancy, for with the exception of Sorby’s 
invaluable memoirs on some special points of enquiry, literally 
nothing has as yet been made public which could even serve as 
an introductory guide to the geologist who might wish to com- 
mence the study of the subject. It is therefore with great hesi- 
tation, and only after much solicitation, that the author of these 
remarks has now ventured into print, with the hope that by once 
breaking the ice, others more capable than himself may be 
induced to communicate the results of their researches on the 
same subject.* 
In the present communication it is not intended to go into 
the details of any special microscopic investigation, but, as far as 
the space at disposal will allow, to attempt a short sketch of 
some of the results already obtained, in order thereby to illus- 
trate the use of the microscope in similar enquiries, and to place 
the same before our readers in as plain and untechnical language 
as possible. 
When applying the microscope to the examination of rock 
structure and composition, it is necessary to prepare the speci- 
mens previously, in order to be enabled to make full use of 
transmitted light in their investigation, since a mere inspection 
of their outer surface, viewed as an opaque object, although 
sometimes of considerable value, does not, however, give a tithe 
of the information which their examination by transmitted 
light will afford. 
When in sufficiently thin splinters or laminae, by far the larger 
proportion of mineral compounds allow light to pass through them 
with more or less facility, and amongst these, most silicates, 
chlorides, fluorides, carbonates, sulphates, borates, and other 
salts, as well as many oxides, and some few sulphides, sulph- 
arsenides, &c. On the other hand, all native metals, alloys, and 
most of their combinations with sulphur, arsenic, antimony, &c., 
along with some few oxides and other compounds, are opaque, 
even when in the thinnest laminse, and consequently when pre- 
sent, as they often are, in minute quantity in rocks, although 
sometimes recognisable by their external crystalline form, are 
* In 1852 the author commenced the study of the microscopic structure 
of rocks and minerals by means of sections prepared by Ochatz of Berlin, 
but soon after commenced the preparation of the sections himself, in which 
he was under many obligations to the kind aid and advice of Mr. Sorby. 
At present his collection of sections of rocks, and their constituent minerals, 
amounts to above two thousand, and represents a wide geographical dis- 
tribution. 
