156 
NoRMANj on Diatomace(S. 
quality of definition over the double system of reflecting 
prisms that has been proposed; the greatest thickness of 
glass that the rays have to pass through being only '096 or less 
than one tenth of an inch. The drawback to the adoption of 
the prism I have described is the great difficulty attendant 
upon its construction. 
The binocular microscope, in its principle to the present 
time, however carefully constructed, gives but little reason 
for expecting any particular discovery to result from its aid, 
as it fails in giving a distinct definition of test objects under 
the highest powers, and must ever fail in this particular, 
while only one ^«//' tlie object-glass is used in producing each 
image. With low powers, however, on objects having an 
appreciable thickness, its performance is sufficiently satisfac- 
tory. In the instrument I now exhibit, all the adjustments 
being fixed, it is not liable to get out of order, and will bear 
as much rough usage as any microscope. The double body 
can be fitted to any of the ordinary forms of microscope- 
stand. 
List of DiATOMACEiE occuvrmg in the neighbourhood of Hull. 
By George Norman, Esq., HulL 
(Continued from page 71.) 
PiNNULARiA, Ehrenberg. 
P. nobilis, Ehr. — In fresh-water ditches not common. Ditch 
Cottingham. Road. Peat Deposit, Hornsea. 
P. major, Sm. — Frequent in fresh-water ditches. Harrogate. 
Hornsea Peat Deposit. Market Weighton Canal. Spring 
Ditch. 
P. viridis, Sm. — Very frequent in fresh water. Spring Ditch. 
Cottingham. Saitersgate. Nettleton. Skirlaugh. Drif- 
field. Eeservoir Waterworks. Haltenprice. Hornsea 
Deposit. Beverley. 
P. oblonga, Sm. — Frequent in fresh water. Cottingham. 
Spring Ditch. Skirlaugh. Hornsea Deposit. Pure in 
Springs near Cottingham. 
P. cardinatis, Ehr. — Not unfrequent in the Hornsea Peat 
Deposit. 
P. distans, Sm. — Very frequent in Ascidians. Sands at 
Hornsea. Dredgings off Flambro^ Head. 
