TTIE LAST DEtADE. 
390 
practical application had been restricted by tlie difficulty of obtain- 
ing crystals sufficiently large and transparent to be cut into appro- 
priate sections, so that the properties of some of the connnonest 
minerals are very imperfectly known. The methods described by 
Dr. Sorby appeared to overcome these difficidties. If a pure, 
transparent crystal be merely magnihed, little or nothing can be 
learnt, but if instead of viewing the crystal itself, it be looked through 
with a suitable magnifying power, at some appropriate object, more 
facts of interest and importance can be learnt than by any other 
single method whatever. The object Dr. Sorby examined through the 
crystals was the image of a small circular hole or of rectangular lines 
ruled on a piece of glass, formed at the focal point of a well-corrected 
achromatic condenser, fixed below the stage, and so arranged tliat 
the image is placed either just below^ or just above the lower surface 
of the crystal. The divergent rays passing through it to the object 
glass are bent, so tliat the focal length is, as it were, increased by an 
amount depending on the thickness of the crystal and its refi'active 
power. The method of testing these values was treated at consider- 
able length and reduced to mathematical formulae. The data thus 
obtained are so remarkably characteristic that they alone would 
amply suffice to identify a large proportion of natural minerals. In 
many cases all the necessary observations can be made with small 
crystals in their natural state, which alone is a very gTeat gain for 
practical mineralogy. The chief value of the method is, however, 
that portions of minerals, of microscopic size, may be identified in 
sections of rocks as thin or even thinner than l-400ths of an inch, 
with an amount of certainty that leaves little to be desired. 
The Geological History of the Strata of East Yorkshire formed 
the subject of an admirable paper by the Rev. J. F. Blake. Yorkshire 
may be broadly divided into two parts by the great Triassic plain 
running north and south, midway between the Penine Chain and the 
sea-coast. To the eastwards of this plain the formations consist 
mainly of lias, oolites, and chalk. The liassic beds in all probability 
were derived from the denudation of the coal measures, and their 
varying characteristics of fine clay or more sandy deposits depended on 
the source from which the materials were derived. AVhen the coal 
