6S8 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
ease with which changes occurred in the frail phosphate and borate 
glasses causing the dimness known as the disease of the apochromatics. 
This was always freely corrected without charge by the makers, and 
it is believed that the glass as now made is free from this defect. The 
fluorite, however, must be taken with its natural defects ; and its easy 
cleavage renders it very fragile. In fact, it is now known that, more 
than thirty years ago, Charles A. Spencer, of Canastota, N.Y., recog- 
nized its tempting optical properties, and used it in the construction 
of objectives, one of which is still in existence; and that he abandoned 
the method because of the early deterioration from the cracking of the 
spar. That now used in the apochromatics is not only clearer and 
remarkably free from colour, but is evidently more durable, and, it is 
hoped, practically permanent. The lenses are, however, absolutely 
limited in output, as the world’s supply of suitable spar is confined 
to the stock now in hand, with no known means of replenishing it. 
It is, therefore, none too soon to be finding some method of making 
high class work without it ; and this is being attempted, with encour- 
aging success, by several makers, in the construction of the semi- 
apochromatics, made from the new optical glasses, but without the 
fluorite. Though not quite as free from colour as the apochromatics, 
their views are very distinct, and their working qualities extremely 
good. The resolution of A. pellucida in ‘ beads ’ by a semi-apochro- 
matic is claimed to have been accomplished in Italy.” 
The Microscope’s Contributions to the Earth’s Physical History.* 
— Prof. T. G. Bonney, in the Bede Lecture for 1892, takes for his text 
the progress in geological research which has been directly due to 
the revelations of the Microscope. By the side of the epoch-making 
discoveries of Darwin and Wallace, Bunsen and KirchofF, he places the 
work in a humbler and more limited sphere of Sorby, who, in 1856, first 
described the results of microscopic investigations into the structure 
of minerals and rocks. The method employed by Sorby was, strictly 
speaking, not novel, for years before, in 1827, William Nicol of Edin- 
burgh had made sections of fossil wood sufficiently thin for microscopical 
examination. The device, however, had not been generally applied, 
and to Sorby is due the credit of first pointing out its wide possibilities. 
Before telling the story of microscopical research into the history 
of the earth’s crust, the author indicates briefly the mode in which the 
Microscope is used in the examination of minerals and rocks. Slices 
are first cut by the lapidary’s wheel thin enough for most of the minerals 
to become translucent, if not transparent. These are then examined 
under a Microscope of special construction, furnished with Nicol’s prisms 
and other optical appliances. 
Upon the history of the two main groups of rocks the Microscope 
has thrown much light. For the igneous rocks it has simplified the 
classification and determined the mutual relations ; while for the sedi- 
mentary group it has shown the true nature of their constituents and 
pointed out the sources from which they were derived. But it is in 
helping to elucidate the problem of the metamorphic rocks, of which 
much less was known, that the Microscope has been of the most service. 
* Nature, xlvi. (1892) pp. 180-4. 
