330 
POPULAR SCIENCE REVIEW. 
MICEOSCOPY. 
Under this heading we have little of any interest to record of work done 
during the past quarter. Hartnack’s immersion lenses appear to he getting 
into general use for work which does not require such high powers as the 
io objectives; hut the subject of immersion lenses requires to be 
more fully worked out than it has been. We want the experience of those 
who have employed both forms of object-glasses, and who can fairly and 
fully state the respective qualities of the two forms. 
A Neiv Form of Condenser has been described in the last number of the 
Quarterly journal of Microscopical Science. By the intersection at right- 
angles of two equal and similar half-cylinders, whose flat sides are in the 
same plane, a solid of a particular form is obtained. Dr. W. Robertson, of 
Edinburgh, who has described this solid, says that were such a solid made 
of glass, and placed below the stage of the microscope, with its square side 
uppermost, rays entering its curved surfaces in directions parallel to the 
axis of the instrument would all be focalised into two lines, or narrow 
spaces, intersecting each other at right-angles. The light would increase in 
intensity towards the centre of the field. By stopping off a diagonal half of 
the square side, I think that a form of illumination would be obtained well 
adapted for exhibiting at the same time the longitudinal and transverse lines 
of PI. fasciola, Nav. rhomhoides, &c.’’ 
MINERALOGY, METALLURGY^, AND MINING. 
The Exhaustion of our Coal . — Still harping on this string. Professor Stanley 
Jevons delivered a recent lecture at the Royal Institution. After pointing 
out the dangers of our present position, and commenting on the impossibility 
of substituting electrical forces for power derived from combustion of coal, 
he States the following conclusions : 1. The power of coal is extending 
itself, and making itself more widely and deeply felt every day. It is more 
and more taking the place of wind, horse, or manual power, and is becoming 
the universal assistant. — 2. We are naW’ally led every day to extend our 
consumption of so invaluable a substance, and experience shows that the 
more we use the more extensive are our augmentations. — 3. Our consump- 
tion is already commensurable with our total supply ; that is to say, we can 
form some notion how long our supply will endure with a stationary con- 
sumption. — 4. As this consumption increases by multiplication, our national 
life becomes shortened, and it is apparent that the increase cannot go on 
very long at the present rate. — 5. The moment we are forced to draw in, 
other nations, possessing far more extensive fields of coal compared with 
their annual consumption, will be enabled to approach and ultimately to pass 
us— 6. The exhaustion of our mines, as it will probably manifest itself 
within the next hundred years, will consist not in any stoppage of supplies, 
but an increase of cost, and the impossibility of increasing the consumption 
each year, as at present. 
